This is literally just porn.
"Oh fuck, oh god, Chekov-"
"Pavel, fuck, fuck-"
Pavel's legs tightened around Hikaru's waist as Hikaru leant down and gasped his way into Pavel's open mouth, tongue desperate and searching, hips hard and rhythmic. Pavel pulled him closer, one hand tight in Hikaru's hair, the other digging into his back, clinging, nails scratching, slipping through sweat. With every thrust Pavel cried out, mouth slack and if that wasn't the hottest thing that Hikaru had ever seen in his life.
A hard thrust and Pavel gasped, his eyes clenching and his teeth clamped, hissing through them, but he moaned and bucked all the same. His lips, so swollen and pink from Hikaru's attentions, begging to be licked, sucked, bitten, claimed, all over again. Hikaru moaned, licking his own lips before moving down to Pavel's pale stretch of neck, nuzzling the skin before tasting his sweat and sucking just below his ear, drawing even breathier gasps from the boy, incoherent mumblings and high pitched moans and he writhed, arching closer to Hikaru's heat, eager and needy.
"God, Pavel," Hikaru panted, voice husky and deep, his words fanning out over the creeping flush on Pavel's chest. "God, I love you."
Pavel's eyes shot open, wide, the green so bright it was almost glowing in the dim light surrounding them. His gasps stopped short, slowing into shuddering breaths, forcing themselves from his lungs and his arms and legs lost their grip.
Hikaru lifts his eyes from the sucked bruise blooming over Pavel's throat to his eyes, his own just as wide, realisation dawning. His own movements had stopped and they were stuck, staring, breathing heavily, Pavel's legs around Hikaru's knees, arms around his neck, Hikaru supported by his hands on either side of Pavel, neither of them able to look away.
"Shit, I'm sorry, fuck, I'm such an idiot, oh god, you know that I didn't, I was just, shit," a blush started spreading over Hikaru's cheeks as he rushed through his words, tripping over them. "Argh, fuck."
He stretched back to pull out but Pavel's arms held him down, legs tightening again, pulling him back in, sucking a moan out of Hikaru.
"Pavel, what are you-"
"Shh," Pavel's eyes hadn't left Hikaru's and looking at them now, Hikaru could see that the shock had left and the lust seemed to have seeped back in, pupils dilating, the thin strips of gold-flecked-green vibrant. Hikaru drank it all in, Pavel's chest, rising and falling with each heavy breath, arousal-pink, crawling up to his neck and definitely his ears, the same shade they had turned when he realised that Hikaru was going to go down on him earlier that night. Despite his self-consciousness, Pavel didn't break eye contact, he just relaxed his grip around Hikaru's waist before clenching again, pulling him closer harder this time. "Just fuck me Hikaru."
Dropping back down, hands either side of Pavel again, he leant over him. His hair hung from his forehead, sweat dripping to land on Pavel's nose.
"But I, and I-"
Pavel licked the drop of sweat as it rolled over his upper lip.
"-I didn't mean and I, you know that and-"
Pavel rolled is hips rhythmically, seductively. Hikaru clenched his teeth, breathing out through his nose. Pavel's fingers twisted in the hair at the base of his neck as he started pushing himself, breaths coming faster again.
"I know! Just - fuck me already!"
Hikaru's growl tore from him unintentionally as he pushed Pavel down into the mattress, lifting a hand to push Pavel's sweat damp curls from his face, propped up with the other as he ravaged Pavel's mouth again, tongue and teeth clashing and twining together, saliva mixing with salt and sweat.
"Harder," Pavel gasped into his mouth, accent so thick his words almost undecipherable.
Hikaru leant back and grasped at Pavel's calves, pulling them up over his shoulders thrusting a few times, sliding even deeper this way, before returning back to Pavel, sharing air. One of Pavel's hands clenched in the sheets as the other reached for his own arousal before Hikaru laughed and tapped his hand away.
"No. Just me."
Pavel whined and his movements became erratic, arching closer to Hikaru, meeting his thrusts, his heat trapped between them, slick with sweat and pre-cum. As he felt the tension build he couldn't stop the stream of Russian that flew from him, garbled and breathless.
"Harder," he panted again and Hikaru dutifully picked up the pace. "No, harder."
Hikaru met his demands, moving harder and faster until neither of them could do anything but pant in the space between them, the air sticky until he knew he wouldn't hold out any longer. Reaching for Pavel's erection, he barely touched it before he released, crying out almost as though in pain as he arched further into Hikaru, smearing it between them, dirtying them. Tighter, Pavel became oh so much tighter as he came and Hikaru let go, reaching his own climax, driving it deeper into Pavel as he shouted out into the Russian's neck, before slowing.
Without pulling out they lay there for a while, catching their breaths and drifting through their post-orgasm haze. Hikaru rolled off of small Pavel, so as not to crush him. He watched him, his breathing slowing, covered in his own cum, debauched. Another blush rose to Pavel's cheeks.
"Stop staring at me," he said through his smile as he turned on his side to face Hikaru. "It's creepy."
Hikaru's laugh was loud and deep and Pavel's easily followed. They watched each other for a moment, their gazes becoming more serious before Pavel reached a finger to his stomach and swiped his finger through his cooling cum. Without saying a word he raised it to Hikaru's mouth. Hikaru just looked at the finger and then back at Pavel, not opening his mouth. Deadpan, but trying not to reveal the broad smile that he knew was coming. Pavel looked at his finger and then back again, laughing.
"I dare you," he laughed playfully, eyes twinkling. Hikaru couldn't fight it and grinned at Pavel, but didn't say anything. Pavel's own gaze sharpened, curious, the eyes he used on a new science project or an equation he had reached a tough spot on. When he spoke next his voice was lower, huskier.
"I double dare you."
In regards to Hikaru's slip-up and the ending, this'd be early in their relationship or they might not even be in one. But Pavel's an intelligent kid and isn't offended when Hikaru tries to explain himself, he knows that they're doing. He's good-humoured about it :')