In an alternate world, where Eggman was indefinitely disabled, and ceased to be a threat to the world, peace allowed Sonic and the gang to kick back, relax, explore hobbies, and discover new interests... 

With no major threats to the world, Tails found himself building fewer weapons & attack vehicles, and more...domestic engineering projects. Planes, ships, cars, and plenty of contract-based jobs. The fox made a nice little profit, and reinvested it into study in industrial & product design, broadening his opportunities. Eventually his best friend, and big brother figure, moved in with him, and helped around. 

One day, a some rather curious product orders came in: sex toys. Tails, being a teenager at the time, kept the work under wraps, even from his roommate. Tails wasn't a COMPLETE stranger to the concept of sex, porn, or such toys, at that point, but this really opened his mind up to it. So, he worked on it, doing his research, drafting, and hiding it as best he could from Sonic. 

Unfortunately, it seems he noticed anyway. Perhaps it was this design left out, or that webpage not cleared from the history of the one computer they shared, but eventually he found more browsing on that computer, even after clearing the history for Sonic. And he never bothered to clear his history, and it looked pretty good, too. Eventually, he decided to get computers for both of them, and their pornographic exploration continued separately, but both flowered. Tails completed the order, and several others, but now he had gotten interested. Retro, hentai, human, or anthro. Even some males. Before long, he found he could suck himself, thanks to all those times he curled into a ball. Simultaneously, and independently, Sonic made the same discoveries. 

Of course, these experiments were strictly kept within each other's personal rooms, and the two didn't discuss it much, if at all. Both were certainly aware of each other, but neither pressed the issue, unless the other brought it up. For his eighteenth birthday, Sonic even granted his wish, and took him to a strip club. It was everything he hoped for, and though they didn't do anything extra, it awakened something in him. He began to take more...explicit projects. And tried a few other products for himself. Experimenting with toys, clothes, cosmetics, and other curious things. 

Around the same time, he and Sonic were not only exploring their own interests, but sharing with each other, more. Not everything yet, but they could converse about it, and actually be rather comfortable with it. At some point, one of them challenged the other to a game of chicken, where the two of them watched a porn video together. In the same room. On the TV. 

And then the video started, finished, and neither had budged from the couch. After that, they weren't sure what to feel of it, but didn't mind a few more rounds over the next few weeks. Eventually, it became a regular thing for them to sit back, and watch some porn together. The two went through entire films, and the two slowly became less modest about how covered they were, or whether or not they had an erection. 

After another breakthrough, where they decided to just up and show each other their junk. A new bond began to form between them. A sort of sexual trust. They hadn't had sex, yet, and it wasn't brought up. But they grew more comfortable with the looks of each other's bodies. And were able to pleasantly chat over the smut. 

"Oh... squeeze those titties baby..." 
"And shake 'em. Make 'em bounce." 

"Yeah, get the oil out~" 
"Please just let it trickle out...just like that~" 

"Hey, he's got a knotted dick like yours, Tails." 
"Yeah, but looks like mine's a bit bigger~" 

"Woah...shit. He's...kinda hot..." 
"And that cute outfit...what's that top called...?" 

They could watch, with their members out, and enjoy the programs to full effect. With both their hands, their mouths, and whatever interesting things Tails got, or made. 

Finally, towards the end of one session, thoughts that Tails had begun to ponder bubbled up. And just about as Sonic was finishing up. 

"Hahh... Oh, god, I'm getting there! Yeah, babe... Stroke that... Stroke that, like it were mine... Fuck, wish we had someone, to..." 
"Hhh... Hey Sonic... You think I could do that for you?" 
"Whuh-Ahh...!?" 

And the hedgehog came, right there. While the idea certainly appealed to him, he appeared to make excuses. 

"Uh, maybe another time... I mean, I guess. But, well... I think this might be it for me, tonight. I'mma go shower off." 
"But you normally cum two more times than this! What's the matter?" 
"Well, I guess it was just that really cute boy, in the purple. He got me going harder than usual." 
"Or, was it just because he wasn't me? Sonic?" 
"Can't hear you! turning on the water!" 

The two thought about it for what felt like hours. They'd known each other since they were practically kids. They were best friends, lived together, and now watched porn together. Was this the next step? Or rather... Where was this leading? Were they becoming... an item? Sonic had special therapy to break up with Amy, or rather reject her. No, it didn't seem like that. And it didn't seem like either of them were gay, the fact of hot chicks getting them both up was proof enough of that. No. Some things had changed, but many others hadn't. Just needed to break through one last wall, to be sure. Tails had just the idea how, and geared up for it. 

After finishing, Sonic had to have gone through several rooms, looking for his buddy. Tails however, waited in the hedgehog's room, having dolled himself up. Once he walked in, that towel got a tent in seconds. Tails had dressed himself up in girly, purple lingerie & makeup, just like the actor in the film, and presented himself on the bed, his ass pointed straight up.

"Tails... I...I'm sorry. I shouldn't have been so..." 
"...So...what, Sonic...?" 
"...Ah, I'm not even sure what the term is...Just too horny, I guess..." 
"And...?" 
"...Come here, cutie~" 

That felt like everything Tails needed to hear, and he wasted next to no time getting to his pal, getting a good look over it all, and getting his lips right around the tip. The fox went right to work. 

"Ahhh...F-fuck...Tails...It feels so weird, from another's touch...it's awesome...!" 
"...And yours...tastes like a whole different flavor to mine. I love it~" 

It wasn't long before he came. But, they had yet to climax. 

"Tails... please... Get on the bed...I need to fill that cute, tight butt of yours..." 
"Oh my fucking god, YES...~" 

Sonic took him, and filled both ends with his seed. The two climaxed together. Became one with each other. And yet, when both had caught their breath, and looked into each other's eyes. They simply saw... 

"My best friend." 

The two slept together, would watch porn together, and kiss & fuck together. But their emotional relationship remained the same as it always was, regardless of their new physical one. Best friends, roommates, and fuck buddies forever.