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Izuku’s always had an oral fixation. From the time he could walk, everybody's always had to keep an extra close eye on him. He's the type of kid that'll stick anything in his mouth if it's not bolted down. The kind of kid that'll lick his way up an escalator and suck on doorknobs if left to his own devices.
They never really get around to weaning him off his pacifier. It's the only thing they have that'll mostly keep his mouth off everything else. But if something particularly interesting catches his eye, Izuku is quick to let the plastic fall out of his mouth as he goes seeking his new prize.
He’s prone to sore throats and little colds, a side effect of always shoving unsanitary things in his mouth.
Thankfully, they all have Katsuki. He's always been a little bit more responsible, more developed, and a whole lot more committed to cleanliness. When he's around, they can let the two boys look after each other, and Katsuki can be trusted to stop Izuku from shoving anything particularly disgusting into his mouth.
He’s also good about getting Izuku into the bath. Or at least, coaxing him in when Izuku is adamant that he doesn’t want one. Usually, his fussing dies down the moment he sees Katsuki splashing in the suds without him.
Izuku usually takes his pacifier in with him, but this time, Inko is a bit more determined to get her son on the right path. She plucks it from his mouth the moment she gets him naked, booting him into the washroom with Katsuki with a pleading look at the blond boy.
“Please, just make sure he doesn’t get anything stuck, okay sweetheart?” She says, probably thinking of the time they’d had to go to the hospital to dislodge a rubber duck from Izuku’s mouth. Thankfully he hadn’t been sick at the time and had been able to breathe through his nose. Inko doesn’t like to think about what could have happened if he’d been a little stuffed up.
“Yes, Aunty.” Katsuki says responsibly, looking up from his perch on the little stool pressed up against the wall. He’s busy scrubbing at himself but pauses long enough to give her a mock salute. It makes her laugh, which makes Katsuki feel like a responsible big kid.
Izuku shoots one last pouty look at his mother before finally toddling over to him. Katsuki can already see his eyes roving around, looking for something, anything to stick in his mouth. All the little toys and trinkets are gone though, leaving only shampoo bottles and the like. Izuku knows firsthand that he doesn’t want any of those in his mouth. Soap tastes gross.
“Stop being weird. Get clean so we can get in the bath.” Katsuki says. He’s only a few months older, but he really, really feels it in these moments.
Izuku predictably pouts, getting a little closer before plopping down on the wet tile in front of Katsuki. The blond tries to ignore him the best he can, spraying himself down and getting all the soap off him.
“If you don’t hurry – ”
"You know, Kacchan your thingy kinda looks like my soother," Deku says suddenly, gaze laser focused on the tiny little cock nestled in between Katsuki’s legs. Katsuki frowns at him, but Izuku’s already sliding across the tiles, eager to make his way over to where Katsuki is sitting on their little shower stool.
"Deku, don't," Katsuki says, a weird look on his face, hands outstretched to keep Izuku back. "You're not supposed to put that in your mouth."
"Who says?" Izuku asks, already fitting himself into the space between Katsuki’s legs. He presses his face up to Katsuki’s dick, tongue already lolling out to suck the small appendage into his mouth. Katsuki makes a strange, choked noise, but Izuku’s already there, already sucking, and the hands pushing ineffectually at Izuku’s tiny shoulders instead moving to lace through his dark green curls.
"S – S’dirty." Katsuki mumbles, head thudding back against the wall as is legs fall open wider. He’s never felt anything like this before. Izuku’s tongue swirling around his tip and suckling at it makes his whole body feel hot and tingly
"Oshn't taesh err-ee" Izuku mumbles, mouth a little full. Katsuki’s privates feel so good in his mouth. Soft and smooth and just beginning to grow in size! And wow! Isn’t that so cool! Izuku is delighted at the physical changes. He loves having his mouth full and this is so much better than his pacifier. This is getting bigger in his mouth the more he sucks. Amazing.
Izuku hums happily around Katsuki’s cock, eliciting more noises from his best friend. Katsuki tugs on his hair, face flushed and breathing heavy as Izuku plays with him. A part of him knows they shouldn't really be doing this. He’s not sure why, only that it's dirty and wrong and they could get in trouble maybe. But he's helpless to put a stop to it. It feels so unbelievably good, he never ever wants it to stop.
They sit like that for minutes, maybe longer: Izuku happily sucking at him, wet slurping noises only broken up by Katsuki’s occasional moans and panting. His legs start twitching of their own accord, and Izuku helps still them by pressing them wider apart and then pushing at the underside of his milky thighs.
Katsuki’s feet go up with a little help from Izuku, lifting them so he's spread wider with his calves resting on Izuku’s boney little shoulders. Katsuki hooks his ankles, and Izuku just sinks deeper onto him, mouth opening wide enough to take his balls in as well. It’s a tight fit, but Katsuki is so far gone for the feeling he doesn't care. He doesn't care that it's his job to keep things out of Izuku’s mouth, not shove things in there. All he cares about is the wet suction around his cock and the happy noises Izuku keeps making, sending shocks of electricity up his spine.
Katsuki’s eyes roll back in his head as his hips weakly thrust up into the tiny mouth working him over. Izuku’s hands have drifted up to rest on his inner thighs, squeezing and kneading at the soft skin there as he plays with his new favorite thing.
Katsuki tastes sweet and good, and Izuku doesn’t think he ever, ever wants to take him out of his mouth. Maybe if he’s good and promises never to put anything else in his mouth, Katsuki will let him suck on him forever.
He’s so busy toying with Katsuki’s cock, running his tongue against every ridge and fold he can find. Izuku doesn’t know when his eyes drifted closed, so focused on hollowing his cheeks, but a soft, needy cry from Katsuki makes him blink them open again.
He’s treated to the sight of his favorite person in the whole world, cheeks cherry red and tears gathering on the dark sweep of his eyelashes. Izuku doesn’t often think things are pretty, but he thinks it about Katsuki right now.
“D-Deku feels like – nngh – feels like somethings g – gonna come out!” Katsuki moans, hips twitching and stuttering against him. Izuku just rests his little hands against Katsuki’s tummy, massaging the soft skin there and sucking in earnest.
He wonders what’s going to come out. If Katsuki might pee. He doesn’t know why the thought sends such a strange little shiver down his back, but he doesn’t think about it too hard.
He draws off just long enough to nuzzle at the wet, hard flesh. Katsuki’s cock is standing straight up, a thin line of drool connecting the tip to Izuku’s lips. He thinks that this part of Katsuki is very pretty too, and he feels hypnotized by the back and forth sway of Katsuki’s bright pink tip.
“It’s okay. You know I like anything in my mouth. ‘Specially if it comes from you, Kacchan.” Izuku says, letting his instincts pull him back and slurping Katsuki back into his mouth with one long, deep suck. The boy in front of him starts trembling in earnest.
“Deku, Deku!” His wails make Izuku feel all warm and fuzzy inside, and he gives Katsuki’s tummy one last, firm little press before Katsuki’s whole body is freezing up – spine going ramrod straight as his dick spazzes in Izuku’s mouth.
Nothing does come out, save for the tiniest little trickle of liquid that doesn’t taste very much like anything at all. Izuku tries not to be too disappointed, just keeps licking and sucking at Katsuki until his wails turn into quiet little sobs and he’s pulling at Izuku’s hair in earnest.
“Stop, Deku stop.” He whines, and Katsuki goes all boneless and weird, sliding off the stool to splay himself out on the tile. It’s not something he’d usually do, but he feels like he really, really has to lay down right now.
Deku finally lets Katsuki’s softening cock slip out of his mouth. He wants it back in almost immediately, but the strange aftershocks traveling through Katsuki’s body have Izuku thinking he should maybe hold off a little.
Instead he just buries his face into Katsuki’s crotch, nosing at his privates and letting his tongue lazily lick at his balls, kissing and sucking at the skin there until Katsuki starts crying again and pushes him away for real.
“I – I said stop!” He yells.
“Sorry Kacchan.” Izuku says immediately, gaze still fixated on the limp little replacement-pacifier between Katsuki’s legs. “Are you okay?”
“I’m fi – ”
“Boys? Everything okay?” Inko calls out from the other side of the door.
“We’re fine!” Two sets of voices call back in unison. Izuku doesn’t look away from Katsuki’s privates, just slinks in close again and kisses the soft skin of his thigh.
“Let me suck on you again later.” He begs, pulling Katsuki’s skin into his mouth and relishing at the little shivers and moans slipping out of his friend. Katsuki looks like he wants to push him off, but Izuku is being good, he’s not touching his cock, just the smooth, sensitive skin around it.
Katsuki gnaws on his lip for a moment before finally nodding. “Okay. But only if you promise not to put anything else in your mouth.”
Izuku perks up immediately, diving in and giving Katsuki’s cock a quick lick before scrambling up.
“I promise! If Kacchan lets me keep him in my mouth, I won’t even need to use my pacifier anymore.”
“How are you gonna get to sleep, dummy?” Katsuki asks, letting Izuku pick him up off the ground.
“I can suck on Kacchan.” Izuku says very simply, all smiles as Katsuki turns a deep shade of red and swats at his shoulder.