After a successful first hang out session, Mikan and Lemi figure its good enough for them and unanimously decide to follow through what they talked about over text, that being inflating in Mikan's shower and possibly more depending on how they feel alltogether.
The boys both somewhat nervously get up from the couch they were just chilling at and talking, and slowly make their way over to the shower not so far away.
While Lemi is nervous and noticably shy, Mikan seems to be more excited about the whole thing, this excitement showing as soon as they close the door behind them in the bathroom.
Mikan quickly moves behind Lemi and hugs him, surprising the birb a little. "Huh? O-Oh um, is everything alright Mikan?" "Yeah! Just wanted to give you a hug, s-sorry if it was a bit too sudden ahah"
Lemi shakes his head and turns around in Mikan's embrace to give him a proper hug. "There! Isnt that better?"
Lemi giggles and lets slowly go off of the proto. "Yeah eheh, well anyways, how about i help you get out of those clothes?~"
Lemi nods, and as soon as he does Mikan yanks the shirt off of him, leaving the birb a little surprised and his feathers ruffled, Lemi quickly tidying them up a bit again as he blushes, Mikan himself enjoying the view of the birb's now exposed chest and tummy, Lemi's chest surprisingly not being as fluffy as one would imagine, instead, Lemi's slight muscles were quite visible, and yet the birb was still trying to subconciously cover and hide them out of shyness. "Hmph! Stop staring at me like that and get naked too!!~" Suddenly Lemi yanks the simple shirt off of the proto just like Mikan did earlier to him, leaving the Protogen without any clothes, which ultimately didnt matter much concidering Protogen's metal platig and armor covering most of their more sensitive parts, that also being the reason why protogens were allowed to not wear clothes in public places, their metal already being a form of clothing.
Mikan chuckled and gave Lemi a hornier and hungrier look with his tongue hanging out. "Oh so we're taking turns? Alright, then it's mine again!~" 
He goes and pushes Lemi to the ground, sitting on top of him and taking off his shorts and underwear nice and slow, almost teasing the birb who was now struggling to fight back and playfully telling Mikan to stop.
This wasnt enough though, and Mikan simply slid the clothes off the birb's legs, Lemi's slit imidietly getting to feel the cold air of the bathroom around them and making his whole body shake. "G-Gah! C-Come on my slit's sensitive! Ya can't just let it suddenly out in the cold like this!~"
Mikan pretty much ingores the birb as he sits on him, Lemi wondering why before he suddenly feels a warm throb on his chest.
It appears the proto let his member finally out as well, it already being well over the need to get hard as it appears it's been teased and edged for a long time now, the tip slimy from all the pre that it keeps leaking out. "H-Hey Mikan? I-I know you're just as excited to be doing this as me, b-but let's take it back a bit, i-i promised we'd inflate, i-i'm not sure if i can do...erm, yeah, anything more j-just yet, let's just get to the inflating, how about that?" Mikan, fairly disappointed and a little embarrassed he let himself get out of control a little, apologizes and gets off, helping Lemi get up.
Even though Lemi just told the proto he didnt wanna do any of that just yet, his tip was starting to poke a little from his slit due to the situation he was just stuck in a moment ago, clearly wanting to move on to inflation so he could mask his arrousal as a reaction from inflating rather than Mikan himself.
Either way, the two give eachother another hug just to establish everything's fine between them and that they can move on.
Lemi turns to the shower corner in Mikan's bathroom and walks over to and into it, getting himself comfy on the cold yet non slippery floor of the shower. The proto quickly follows, and first thing he does is get the hose and start spraying it on the ground after closing the door to the shower, finding a good temperature to use.
After a little bit of setting up, Mikan aims the hose at Lemi and sprays him with it just to playfully confirm the wamrth was just right.
Lemi nodding and taking the hose from him as he starts to undo the head of the hose so they have easier time using it.
The head comes off and water starts shooting on the ground at high speeds, Lemi looking reassuringly at Mikan, as if to ask if this was the speed he really wanted them to use, and Mikan only giving back a slightly smug looking smile, getting Lemi riled up and determined to take it.
And so he did, he took the hose and brought it to his behind with one of his hands, the other spreading one of his cheeks and letting the warm water shoot at his bare tailhole, making Lemi clench a little and quiver.
With a deep breath, Lemi pushed the hose inside of his tailhole, his tail feathers ruffling up and standing straight up in the air before settling down again seconds later.
Lemi was nervous and shy to be inflating in front of anyone, while he did so before already, it was with air and he got to be dressed, this time he was naked and inflating with water, an inflation he didnt do too often due to its weight he couldnt handle well. Even so, he pressed on and focussed, he did his best to relax and even decided to lean backwards against one of the now warm shower tiled walls, giving himself support as his legs began to feel unstable and shaky.
Water flown inside of him from the moment he probed himself initially, causing his lower guts to feel heavy and hot from the water intake they were currently experiencing. He gasped for air and tried to stay awake with his vision going hazy from the sensation of the water traveling through his guts to his belly. Soon he felt and saw a distention forming in his middle section, a small ball of water growing inside of him, a notable bloat, and it was growing. He took deep breaths, almost forgetting about Mikan ever being there as he was as quiet as a mouse, watching in ave and stroking his member as he watched the show happening rught in front of him.
Lemi blushed when he once more realized he had an audience, and decided to play his inflation up a bit, putting his paws over his belly and letting them stay there as he quietly moaned, the procedual growth  and filling up drifting his paws and fingers apart from eachother, as his moans became more audible and more real by the minute. His bird cock was fully out now, hanging out of his slit and drooling pre all over the shower floor as he held back the tought of grabbing it and wanking off to his inflation already, knowing he'd end his growth prematurely that way.
No, instead he kept his hands on his belly and kept rubbing circles all around it, feeling his stomch getting heavier and more hung as time went on. He felt his insides, making the water belly he's now gained, bubble, making Lemi giggle a little right before moaning loudly due to his form growing some more.
Lemi's stomach was reaching the beachball size, and it was looking strained, just like Lemi's face. He was sweaty and basically falling to his knees at this point, his situation only excigting Mikan even more, so much in fact, that he couldnt wait any longer. As soon as Lemi felt one last growl from his insides, he decided to pull the hose out and clench, a little bit of water dripping out but otherwise his size keeping form. Instead though, Mikan yanked the hose out of Lemi's paw and rinsed it himself.
Lemi, a little confused, watches Mikan on his knees, hugging his hurting waterlogged belly, Mikan cleaning the hose off before smirking and opening up his screen, stuffing the hose down his maw, and chugging away at the rapid stream. In no time, Mikan was able to grow half Lemi's size, rubbing his stonach and smacking it a few times just so the slightly metalic sound of his bulged metal sheets could be heard.
His member looked like was gonna burst, but Mikan somehow was able to keep it under control, instead promptly pulling the hose out of his maw and stuffing it into his own tail hole instead.
Mikan seemed like he wasnt struggling at all, even managing to waddle slowly over to Lemi and squishing his inflating, filling belly against Lemi's strained feathered tum. The stomachs squished together and wobbled with water, still somehow remaining spherical even after all the outside probing and poking from Lemi and Mikan, their eyes locking for a bit as they couldnt help but touch eachothers members with their paws, stroking eachother off as best as they could as Mikan was quickly catching up to Lemi in tum size. 
"Hey...y-you dont have to mnf~ Force yourself as big as i am, y-you know?" Lemi noted, not wanting Mikan to hurt himself, only to get the responce of "I'm not even feeling tight yet, i'm just growing as big as i can manage~" From Mikan. Suddenly, the birb started moaning loudly and continuosly, Mikan jerking him off as quickly as he can while being very close to beating Lemi's size.
Clearly, he was making the birb cum out of pure enjoyment of hearing him moan and seeing him struggle, himself not being in the best position either though.
Just as he gave the last few strokes to Lemi, causing him to cum all over himself, the bathroom floor, and a little on Mikan, who has just reached a little over the beachball size, bigger than Lemi.
He wasnt exactly struggling, but it was obvious he was starting to really feel the weight, himself sitting on his knees just so he doesnt have to hold it up anymore, and letting out loud huffs of tiredness.
Withing the few more minutes, Lemi finally caught his breath and noticed Mikan, still struggling to grow bigger, he opened his beak, ready to tell Mikan to stop, but he saw that the protogen wasnt really in pain, he was just having a hard time growing anymore, leading Lemi to keeping his beak shut and rather rubbing all over Mikan's now spherical belly, Mikan himself reaching behind himself and grabbing the hose, ready to pull it out at any moment.
And there they were, kneeling and waiting, the protogen sweeling bigger, and bigger, tighter and stiffer, until finally, something strained inside of Mikan..
Mikan yanked the hose out of his ass and let it lay on the floor, his member shooting strings of cum in the air, on his belly, and even a bunch on Lemi in return for Lemi doing the same earlier. He gasped for air and moaned loudly with his digital-esk voice, Lemi just giggling and continuing his rubbing motions. 
Mikan, now worn out, lays on his belly and huffs and puffs, Lemi somewhat doing the same as he leans agains his smaller belly. The two of them exchange a few looks and giggles, poking and probing eachother, laughing at their ridiculous situation.
With some time passing, the two decided that its high time they deflated, doing so within the span of a few minutes and cleaning up the shower corner they messed up a little, quickly leaving it and dressing back up in their spare clothes, not before they were able to give eachother one last nude reassuring hug and pat on the back.
As they leave the bathroom, the birb and proto sighed with relief, both of them still a little bloated and weak, but definitely very happy and satisfied. Lemi picks up his bag he came with and heads for the door, looking back at Mikan. "Well um..i should probably go, its getting late and all, so yeah, might be a good idea to do so hehe" "Yeah...but we'll see eachother again, right? I-I enjoyed the night and, i wouldnt mind hanging out again sometime!" "Oh dont worry, i'll stay in contact as long as you do too! I had fun, and i feel like there's more of it to be had, so yeah, i just think we both need a rest now, dont we?" Lemi smiled at Mikan, giving him one last tight hug in the door, and a small peck on Mikan's cheek. "Anyways yeah, see ya and take care Mikan!" "Same to you Lemi, goodnight!"
As Lemi disappears behind Mikan's appartment door, the proto sits down on his couch and sighs, both with relief and glad he could finally relax a little.
And yet, he couldnt help but grab his crotch and rub his yet again hard member, still super sensitive from before, making himself groan quietly with gentle squeezes and strokes. "Heh..still wasnt enough for you, huh buddy?~ Well thats alright, im sure we'll have fun with him again soon, besides, at least we have some pretty amazing fantasies of our new friend to keep us entertained until then, right?~" Mikan started to slowly and carefully jerk off on the couch, legs spread apart and member in hand.
As it got darker and darker outside, Mikan moaned and groaned into the night, huffing as he jerked off to all the ideas he was getting, as to what he and Lemi could do in the future, saying Lemi's name under his tired worn out sighs, he couldnt help himself, and who could blame him.
Both him and Lemi were absolute hornballs, and with them finally knowing eachother, they were still gonna get into many more interesting situations together from now on, they just had to wait and see.