You already know what happens.
Pajama bottoms left in bed, Ron positioned himself over Harry's narrow body. He was naked from the waist down, his knees on either side of Harry's hips, his calves and feet tracing the curves of Harry's cool, soft buttocks. Ron's freckled cheeks glowed red. Was he actually going for this? Did he actually want this? Fred and George were sleeping right behind them. He'd never hear the end of it if they woke up and saw what he was doing. Yet - upon seeing Harry pulling away at himself, and what he was wielding, Ron couldn't deny the overpowering, alien desire in his body.
Harry's emerald eyes were watching him, half-lidded beneath his glasses and mess of hair. Hus cheeks also looked flushed. He looked a little sweaty, even. One of his hands was still gripped around his cock, which was directly below Ron's bum. They watched each other, full of desire, but both hesitant about making the move. Ron's mind raced, feeling himself growing down there. The longer he waited, the more of an urge filled his rump and pumped into his length.
Of course it was wrong, he thought, heart beating. That's what everyone would say - that they shouldn't be doing this. Harry was his friend. Harry was his best mate. And Ron wasn't into getting plowed - at least, he'd never thought as much. Truth be told, he'd never thought about it at all. But this... he wanted. It was undeniable. And somewhere in the back of his mind, there was a voice saying, who better? Who better to do it with, than your best mate? Who better to let them bury their bone in you, than the Boy Who Lived? Anyone would kill for the opportunity. It was a no-brainer!
And he did want it. He was nearly at full mast now. Ron looked away from Harry's eyes, down his torso, at the bare midriff and the tight muscles shown there on Harry's skinny build. Ron grew larger. His eyes moved down to Harry's groin, at the small hairs laying there, then finally at the shining, moonlit, pulsing broom handle gripped in his hand. Ron's arse was empty, and he needed it filled.
Relaxing his back, he let his stomach fall forward. The tip of his penis made contact with Harry's waistline. Ron found it warm. He adjusted his knees and reached back, gripping each of his firm, pale, freckled cheeks in his hands, and lowered further, spreading them. Harry's cock made contact with his taint. They met eyes, Ron's bangs hanging down in his vision. Harry Potter, he thought. Star quidditch player, defeater of You-Know-Who, his best mate. He swelled, his breaths coming in short and he lowered even more, relaxing, the point of contact wandering up… and finding his bunghole.
He was still too tight. He knew he was. He let out a lungful of air, letting the tension in his leg muscles ease off. He relaxed the muscles of his buttocks. He spread his cheeks. And slowly, eased himself onto Harry.
First the tip slipped inside, engulfed in Ron's flesh. Ron felt it in him. Then it was halfway, the shimmering, veined underside slipping inside little by little. It was enormously large. Filling him. Ron wiggled his hips, easing himself lower and trying not to grunt. He still held his cheeks wide. It hurt, a little. It felt like he was constipated, like he had an enormous turd, only it was moving in the opposite direction, and slipping up unstoppably.
Harry's hands moved up to Ron's hips. Ron let go of his cheeks and pulled his shirt up under an armpit, trapping it there so Harry could run his hands up his backside. They were officially doing this, he thought. He and Harry. He eased himself all the way down, until he was sitting atop Harry's lap, and now the entirety of Harry's length was throbbing inside him.
Then he moved. He planted one hand by Harry's head, sliding it under the pillow so his palm was on the cool mattress. He leaned over Harry, Harry's cock sliding back, and that... that felt good. It was like taking a shite, only in the right direction, and a hundred times better. Then he sank back again, Harry filling him once more. The deeper it went, the better it was. Ron's breath caught.
Deciding to lose the shirt, he pulled it up, letting Harry soak inside him, and nimbly tugged it from his head and arms and tossed it at the edge of the bed. The moonlight revealed his pale, freckled back and shoulders. He did a quick check for Fred and George - they were still snoring - so he resumed. He put his other hand beneath Harry's pillow, and sank back again. Harry's thickness slid into him. Ron kept his mouth shut, trying to be quiet, but was sure he was making more noise than he knew. Harry gripped one of his arse-cheeks, and Ron looked back at it, seeing Harry's fingers making depressions in his flesh.
Ron went forward again, pleasure pounding up his body, burning his face. His own cock rubbed up Harry's torso, bunching up his pajama top. He sank back, and Harry, unable to contain himself, lifted and shoved himself in deep. Ron took a sharp intake of breath.
"Harry," he said.
He started rocking faster, gyrating his hips - Harry obviously wanted more. Ron's eyes went to his lips. Should they kiss? They were friends, but he sort of wanted to. He acted on the impulse. Digging his fingers under Harry's shirt, he pulled it up. Harry went along, putting his hands up. Ron tossed it aside, and when Harry's face came out - black, messy hair relaxing over his face - their lips met. Ron kissed his top lip. Harry kissed him back. But then, by mutual agreement, they stopped, deciding it wasn't doing it, and settled back into motion.
He couldn't believe they were actually doing this. He was riding Harry Potter. His best mate was buried inside him, fucking him to new heights. It was incredible.
"Ron," Harry said, and he realized he'd grunted. But the next moment Harry gave a short gasp, turning his head to the side and covering his mouth with the back of his fingers, and Ron smirked, sinking back over him. He reached for his own meat, looking at Harry's bare body, and began to rub himself, pressing his tip against Harry's warm stomach.
Outside the door, Hermione stopped. She was sure she'd heard some noises coming from the boy's room.