Chad stood before the wall of dildos, not sure what to think. That he had even gone into the store at all surprised him. He thought of himself as a pretty normal, midwestern white guy. He'd gone to church with his grandparents as a kid, got into a state school on a sports scholarship, was studying lit so he could teach at a high school and coach. He was an “all-american” kind of guy and yet, here he was looking at neon green and purple dicks for his girlfriend to peg him with.
“Have you got any questions?”
The voice, which was crystal clear, came from behind him, and he whirled around to face who had asked. The clerk was a stout woman with unnaturally bright red hair and brilliant blue eyes. She was also incredibly stacked. So much so that her smock functioned more as a corset than an apron and her tit flesh quaked with each breath. Ink peeking out from the hem hinted at several concealed tattoos.
Chad looked away once he realized his gaze had settled on her freckled cleavage but there wasn’t much in the store to look at that wasn’t equally embarrassing.
“I…I don’t know actually. I mean, I’ve got questions, yes but-”
“Look,” she said putting a hand to her hip. “No judgment here. Everyone’s nervous the first time they come to a place like this. Society as a whole frowns on exploring sexuality” The clerk raised both hands and sighed, setting off a massive boobquake in the process.
“I…Thanks, really, but I think this might’ve just been a bad idea.” Chad turned to leave.
“You love how big anal makes you, don’t you?”
Chad turned slowly. “H-h-how do you know that?”
“Well, you were looking at dildos and you’re obviously not gay from how hard it is for you to keep your eyes off my rack. So I’m guessing your new girlfriend plays with your butthole while going down on you, right?”
“That’s actually pretty close to it, yeah,” Chad admitted, rubbing the back of his neck. “Sara and I have been together for a couple months, but just started being intimate last week. She used her middle finger the very first time, like it was natural for her. I was surprised at first, but I’d never been that hard jacking off before–and we went for what felt like forever.”
The clerk crossed her arms over her chest and leaned towards Chad. “And that was a week ago? How’d you arrive at toys already?”
“We’ve…had sex nearly twenty times in the last seven days. Mostly because even if I’ve orgasmed already, just the feeling of her inside me gets me hard again. Each time we’ve had sex, she’s stimulated me even more than before. Last night, after I took a shower, she even used her tongue.”
“I see. It sounds like you’re really getting into assplay then. Do you want something simple to feel out this new direction or..?”
“I think…I think I want her to fuck me.”
The busty clerk stroked her chin a couple times and then walked back to the counter. She disappeared for a moment as she bent to retrieve something. Finally, she stood up, with a box in either hand. “How thick is Sara?”
Sara, with a cock the size of her arm, flashed through Chad’s mind. “Hu-wha?”
“I mean her pant size.”
“Oh! She’s an eight or a ten, depending on the cut.”
“Then yeah, this harness is perfect,” the clerk said, putting one of the boxes down on the counter. “And this,” she said, handing the box to Chad. “Is a great toy to start off with. It’ll really grow on you.”
Chad turned the mostly white rectangle over in his hand, hoping to read specifications. Most of the packaging was in German, though the numbers were all very small. Was he really going to buy his girlfriend a strap-on? What if she thought he was weird? What if this crossed a line he’d never thought to question?
“Well, are you gonna get this stuff?”
“Sure, why not?”
Chad was oddly eager when he got home. He was already working at the seals on the box as he closed the door. The imitation phallus wasn't much bigger than his own and it felt almost natural in his hand. The hard, teal rubber was formed into a seamless bullet-like shape that was ever so slightly curved. The hint of a rounded ridge cast a shadow over the glistening surface near the tip. The base abruptly flared out into a flat ring, likely so it could be secured.
For some reason, Chad could hear his heartbeat. The steady rhythm grew louder the longer he looked at the dildo. He licked his lips. He turned the shaft over in his hands. What would Sara look like with this on? He was excited to find out.
He was also pretty sure something was wrong. 
Try as he might, he couldn't recall actually stepping into the store or even where the store was. The last thing he remembered was idly entertaining the idea of Sara and him taking her actions to the next level. Come to think of it, had the clerk ever introduced herself? There wasn't a receipt in the nondescript black paperbag with the harness and lube.
He was about to pack the whole lot away when Sara came through the front door.
Something about the situation really made Chad look at his girlfriend. Suspended in time as she crossed the threshold to their apartment, her amber-brown eyes stared out from rectangular glasses framed by braided dark hair that hung past her shoulders. 
Though average in height and build, there was something about her that was inarguably cute. He was willing to admit her toasted marshmallow complexion was what had caught his attention. As the daughter of immigrants from Bangladesh, she stood out among the predominantly pale population on their Midwestern campus. However, it was her irrepressible enthusiasm that had made Chad fall for her.
Her gaze flicked to his hand, then to his face, before a grin spread across her face. “I didn't expect you to move so fast. I was worried I was forcing you.”
“I...I guess so. I mean, it all happened so fast, but the clerk said this one would really grow on us.”
Sara closed the door behind her. “Is it a strap-on? Did you get me a harness?” She rushed over to the bag and let out a little squee as she pulled the box out. She whirled around and snatched the dildo out of Chad's hand.
“I'm gonna go put it on!” and then she was around the corner into the hall that lead to their master suite. For once, her enthusiasm was off putting. Chad had expected her to at least hesitate, but from the sounds of cardboard being torn in the bathroom, there was only one thought in her mind. He resigned himself to a long night.
A moment later there was a gasp from around the corner. “It looks so good!”
The teal phallus came into view first, then Sara's naked body. Her braids were undone, the kinked hair shrouding her torso. Her glasses were absent. Chad's gaze was drawn down, something seemed off. On the box, the harness had resembled a bikini bottom, but it seemed like Sara was wearing a garment that only consisted of heavy metal rings attached to straps that held tight to her hips and thighs.
She caught his appraising expression and laughed before turning around. The belts on her thighs were tight, making her butt seem bigger, especially as she slowly bent over to flash her vag. 
“I took the fabric off, lover boy, I didn't want to be cooped up once I got you started.” She turned back to face him, her hand resting on the shaft. “I don't want to have to take this off.”
“I..I don't follow. Why don't you want to take it off?”
“Explaining it would take far too long. Now, are you going to come over here, or do I have to take charge right away?”
Chad gulped, but shucked his shoes and shirt before walking over to sweep his girlfriend off her feet. She wrapped her arms around his neck and pecked his cheek. 
“Come on, let's take this baby for a test drive.”
He set her down on the bed and then knelt between her legs. He tentatively put his hands to the toy, not sure how his attention to it was going to get things rolling. The feeling from before, the odd need to put his lips to the rubber won out and he kissed his girl friend's fake phallus. Sara leaned back on the bed as Chad's kisses moved up, moaning as if the attention was stimulating. Egged on, Chad began to lick the underside of what would be the head.
“Yes, loverboy, that's it. Show me how much you love my cock.” Sara spread her legs wider, her hips bucking ever so slightly. She really was getting off on just the idea of having a dick. He slid his hands to the base of the toy, his thumbs searching for her pussy. He slipped his digits into her folds just as he took the tip into his mouth.
It was then he could've sworn her felt the toy twitch.
“Okay, lover. Now that you're starting to suck, use the roof of your mouth on the head. Press me into you with your tongue.”
Chad did as he was bid, even as he kneaded Sara's silken walls. With each circle of his thumbs, he slid ever so slightly further down and the back up the tumescent shaft. His girlfriend was breathing raggedly now, as if his tentative blow job was the best thing in the world.
He had to admit it was pretty hot though. His own manhood was straining his pants already. His pulse pounding against his leg. Was he really going to come just from sucking on a fake cock?
“Alright, that should be enough.” Sara said as she sat up. “Strip and let's get this show on the road.”
Chad undid the button on his pants and pulled them down along with his boxers. His cock, freed from the confining fabric, sprang to attention and began to throb.
Sara raised an eyebrow, a mischievous grin on her face. “I didn't think you'd be that hard from just a little teasing,” she said as she rose and strode across the empty space between them.
“Y-yeah...it's a little embarrassing.” 
He looked away, but Sara turned his face back to her as she pressed her chest into his. Her teal phallus was rubbing against his balls, the stiff rubber slick against skin. She cupped his dick, her soft hand caressing his shaft.
“Don't be embarrassed, lover, I know you want me bent over just as much. I know you can't wait to pound my snatch with your iron-hard cock.” 
She kissed him then and the pair spun until Chad's back was to the bed. A little push from Sara sent him sprawling backwards. She lifted his legs and pressed against him, then stopped. He could feel the rounded tip between his cheeks, but it seemed Sara couldn't penetrate him.
“You got lube, right?”
“Yeah, I think we left it on the kitchen table.”
“I'll go get it then, you stay comfy,” and she was out of the room before he could reply. Naturally, his hands wandered to his dick. He squeezed it a couple times before Sara returned with the bottle.
“Right then. For real this time.” She squeezed some of the liquid onto her hand and rubbed them together. Kneeling down, she pushed her middle finger into Chad as she took his cock in her mouth. Almost at once, Chad was his hardest, but Sara didn't stop. Another slick finger slipped into his hole as she sped up her pumping. She swirled them around, pressing against his sphincter. He could feel his body beginning to tense as climax neared. He'd never come in her mouth before, he always finished into a condom inside her.
Sara's fingers bushed his prostate and that pushed him over the edge. He bucked hard into her face, before unloading. To his surprise, she swallowed. 
A moment later, his body relaxed in the throes of afterglow. Sara rose so that the phallus was again between his cheeks. She squeezed out more lube, getting as much on him as the teal toy.
“Are you sure? Last chance to stay vanilla.”
“Yes, I'm sure. I want to know.”
The sensation of the dildo entering made Chad shudder. The rounded tip, aided by the lube, spread him open effortlessly. Even though he had just come, his cock was starting to stand again. Slowly, Sara drew closer, plunging two, three, four inches into him and by time Sara's hips were against his, he was ready to go again. The feeling of being filled was a bit unnerving, but Sara's blissful smile was reassuring.
She let go of one leg, hitching the other up on her shoulder at the knee. Her free hand wrapped around his dick and began to stroke. As she did, Sara started to move her hips, drawing the strap-on out of Chad before slowly sliding it back in.
The twin sensations caused Chad to moan loudly. He could feel himself on the edge again already. His balls were tight against him, his breath was coming faster. Sara increased her tempo, drawing further away each time to push farther, faster. Her own moans mingled with his, egging the pair to push harder.
Without warning, Chad came again, this time spraying all over Sara and himself. It felt like there was even more than before, but he dismissed it as heightened awareness. His orgasm didn't deter his girlfriend from continuing though. Her strapped on appendage would never come, she could fuck as long as she could move and that seemed to be her plan. She was pounding him now. Her hips crashing into his every other second as she pumped with as much of the five inch phallus as she could.
He expected his butt to grow sore, but only pleasure emanated from within. With each thrust, the curved ridge of the phallus was dragged over his prostate, causing wave after wave of sensation to travel through his body. His breathing was growing ragged and sweat was beading on his chest and face, but still Sara pushed on and for a third time in twenty minutes, he was hard.
Looking at his penis, it seemed bigger, thicker. Their eyes met and Sara grinned. She thrust one last time before pulling completely out. She wiped the strap-on with a towel and then slid a condom on him, before climbing onto Chad. She pressed her hands into his chest as she tried to get into position. Chad reached between them to guide her, his arm against the phallus. As she slipped around him, he could swear the rubber dong throbbed. She pumped him a couple times in this position. Each time her hips moved away, the strap-on pushed into Chad's abdomen. He swore he felt it pulsing. 
Sara pushed back into him, her knees squeezing tight to his hips. She sat back, her hands on his thighs, and he grabbed on to her ass to keep her in place as she began to grind against him. Her eyes were closed, lost in the sensations.
Chad however, could only focus on the teal dick attached to his girlfriend. He could swear it was pulsing, that the smooth surface was growing larger with each passing moment. Hadn't it been nearly bullet-shaped before? Was the ridge always that well defined?
He wanted to touch it, to confirm his eyes weren't playing tricks on him, but Sara's tempo increased, her gyrations beginning to shake the bed. She flipped her hair back and something else caught his attention. There was definitely more jiggle up top than he remembered. Before his eyes, Sara's breasts bounced larger and larger. That wasn't all that was growing either. With each motion, there was more flesh between his fingers.
Overwhelmed, Chad felt his body tense as he came once more. Sara's nails bit into his legs as she pushed even harder, before she too tensed with anticipation. She collapsed onto him, breathing heavily. It was unmistakable, there was definitely more of her now than a moment ago and the strap-on was throbbing like mad.
She rolled off of him, her breathing still coming in gasps. The surface of the teal dildo was no longer smooth. Veins coiled around its shaft, vanishing into the harness. The tip was decidedly darker. For a third time the odd compulsion washed over him and Chad swung his leg over Sara's body. Her eyes snapped open as he rested against the phallus, but she didn't say anything.
The false cock was as hard as before, though the inexplicable pulsing made getting it lined up difficult. The tip kept dragging along the space between his balls and his anus. Sara's gasps turned to moans. Finally, Chad felt it spreading him and his girlfriend's body reacted. With one swift motion she thrust up and pulled him down to impale him. He saw stars as the metal ring pressed against his cheeks.  
Chad wasn't sure where to put his hands, but Sara answered him before he could ask. Her hands forced his own to her tits before she smacked his ass and grabbed on tight. In that position, he tried to mimic her motions from earlier, but couldn't quite get his hips to move that way. Whatever he was doing seemed to please her though, as her moans grew louder and her grip on his ass tightened. Beneath his fingers, her breasts had begun to grow again, pulsing ever so slightly large with each moan.
He probably would have kept going like that forever had something not bushed against his arm and made him look down into a valley of cleavage that was growing from his own chest.
“AH!” He sat back, his hands flying to his own, now decently endowed chest. “What the fuck?”

To say his afternoon had take an interesting turn might've been an understatement. What had begun as an idle curiosity for Chad had given rise to a certifiably insane situation. The athlete's hands cupped breasts he had suddenly and mysteriously grown. All the while, his girlfriend Sara was growing as well. The longer she fucked him with the mysterious teal rubber phallus, the more her breasts and hips swelled. The ever stranger strap-on was the center of it all. From the way it was changing, becoming more life-like, he could swear it was coming to life on her pelvis as the throbbing had grown even more noticeable since the first time it pulsed in his mouth.
It'll really grow on you both...
He didn't have time to further ponder that development, as Sara pushed him over and began to slam into him even harder. In the back of his mind, he knew this much action should hurt and yet, the pressure was pleasing. With her hands pressing into his shoulders, her torso hung above him. The motion of her growing bustline was hypnotizing. His own breasts forgotten, his pressed his fingers into her caramel-toned flesh, relishing the motion and the feeling of her now pillow-like aureole. 
With each smack of contact, Sara's assets throbbed larger. “Yes, loverboy,” she said as if chanting, her thumbs digging into his lats. “That's it! Yes. Worship this new body of mine. Worship your new goddess.”
There was a pop and a sudden flood of hot liquid as Chad came again, bursting the forgotten condom.
Sara curled against him, pressing her body into his. She thrust slowly, uncoiling at the same time while dragging her fingers along his body through the jizz. Her nails felt sharp against his skin, setting off sensations of burning. When she sat back, her torso was coated by an impossible amount of milky fluid. It dripped from her breasts and ran down her body from shoulder to hip. The resulting glaze highlighted that her bust and hips weren't the only things changing. It was as if most of the fat on her frame had been relocated, leaving behind a physique that rivaled an athlete in peak condition. 
Chad's cock, itself much larger, twitched once more, releasing another small burst that landed in her now bountiful cleavage.
“How is this happening?” He said it out loud as he though it.
She grinned, dragging her fingers over her breasts. “What does it matter? I'm becoming a divine being of pleasure. I'll be able to live my life the way I want now, without being beholden to everyone what expects me to be.”
Her enthusiasm was frightening and at the same time intoxicating. Even so, Chad wasn't sure he could deal with the level of physical changes they were both undergoing. The strap-on had to be the culprit. Perhaps if he unhooked the harness, the phallus' effect would cease.
For the third time, Sara began to thrust. Her strokes were slower, less fevered, allowing Chad to realize just how much the false penis had changed. It easily pushed deeper than five inches now and its thickness had increased as well, though he couldn't tell by how much. 
He sat forward, reaching for the clasps. Sara wrapped her arms around him, her breasts slipping against his. She was massaging him again, her cumslick hands drawing circles on his shoulder blades. Heat like before spread over his back and sunk into the fiber of his being. Sara's breasts pulsed against him, bit by bit enveloping both their torsos. His own fingers found the harness, but tugging at it just forced their positions to change. 
Chad was sitting on her lap now, his head above hers. His cock throbbed between them, soft boobflesh slowly swallowing it up. She nibbled at his chin as her hands wandered elsewhere. There was a chill on his back as Sara poured out more lube. It ran down the small of his back and between his cheeks as she increased her pace, her hands drifting to his hips to resume their digging massage. 
He fumbled for the clasps. 
“What's this, loverboy? Are you done already even though you're this hard?”
Sara was slamming into him now, the force of her impact shoving his swollen shaft deep into a valley of quivering velvet. 
The snaps came undone. Sara screamed in orgasm. The harness fell away. Warmth filled Chad's insides. Everything went dark.
---
Chad awoke to a dark apartment. Sara was asleep on his chest, pinning his left arm. Her shoulder was buried in the crook of his neck, blocking his view of everything. Was she was still transformed or had she gone back to normal? He didn't want to move and find out. 
As he lay there in the dark, it occurred to him that she hadn't been the only one changed. His curiosity piqued, his free hand moved towards his penis and arrived much sooner than expected. He wrapped his fingers around it. Flaccid as he was, the shaft was still larger than it had ever been while hard. His whole member used to be just longer than his palm. Now, he couldn't feel either the base or the head from where his hand resting. Just how much had they been altered?
Sara began to stir, as if his question had caused time to begin moving again. She pushed herself up, revealing just how much her body had changed. Her breasts hung to just below her elbows obscuring most of her torso. They were capped by curved aureole and thick, stubby nipples. Her As she sat back, Chad's gaze was drawn down her thickened, athletic frame to her crotch, where a caramel-colored shaft emerged before curving down between thighs that were thicker and more muscular than he remembered. It seemed the changes had continued even after he tried to remove his girlfriend's decreasingly fake penis. Honestly, the near amazonian woman in bed with him bore almost no resemblance to the girlfriend he had given the strap-on to. 
With Sara no longer obscuring his view, Chad took in the changes to his own body. He was thankful the breasts he had sprouted hadn't grown into much more than handfuls, but that wasn't all that had changed. His physique had gone from trained athlete to someone on the cover of Men's Health. His forearms and biceps were larger, more well defined. Thick veins stood out through his skin, throbbing noticeably with his pulse. What he could see of his torso was just as ripped, as if he'd been bulking for years. It seemed as if his hips flared outward, but it had to just be a trick of the angle. And his dick! It had to be nearly as thick as his wrist! His balls, too, were larger and their weight in his hand was mind blowing.
“Well, that certainly was a thing that happened.” He said it instead of thinking it as he continued to absently fondle his enhanced endowments.
Sara turned, her expression hovering between hunger and trepidation. “It most certainly was. I didn't think it would go this far.”
“Hang on. You expected this?”
She shrugged, setting off a cascade of quivering caramel titflesh. “To some extent, yes. Sort of. I didn't expect the Goddess formula would work so well and so fast—or frankly, that it would affect you.”
“I don't understand why--” chad began before Sara cut him off.
“My parents don't take me seriously, they disapprove of our relationship, they constantly say they wish I was a son. So...I became one.” She demurely rested both hands on her impressive member. “With this, I can now sire an heir--which is all they care about--and I can live my life unfettered.”
“Except for the whole fact that you're now a physical aberration. How do you plan to go to class looking like that and not get stared at?”
His girlfriend laughed. “Oh loverboy, these changes aren't permanent. We'll return to normal in a few hours and then we can grow all over again!”
Chad's dick twitched at the thought of Sara always becoming this busty amazon whenever they had sex.  He had to admit he wasn't unhappy with the changes to his own body either. He felt powerful like this. In fact, if he was honest with himself, he wanted more.
“What, uh, would happen if we had sex again right now, as we are? Would we--”
“Get bigger? I'm not sure. Like I said, this is already way beyond what I had in mind when I designed the tattoos.” She smirked then, reaching over to pat Chad's quickly hardening cock. “I'm sure we could find out the limits...”