Dean's really gentle the first time. There will be time can't-keep-my-hands-off-you-one-second-more roughness, but no one has ever touched Sam this way. He knows that just like he knew the first time Sam masturbated—Sam's red cheeks and the way he kept adjusting himself when he thought no one was looking, hyper aware of his cock in a way he never had been before, and the way he flexed his right hand unconsciously was confession enough. Dean knows no one has ever unbuckled Sam's belt and pushed him down the same way he knew that Sam's first kiss was with a boy not with Rachel Nayv no matter what Sam said—the way Sam wouldn't look at him and how Sam suddenly became interested in what everyone, from the Bible to scientific studies, had to say about homosexuality was enough to clue Dean in. Just like he knew then, he knows from how Sam clings to him, how his breathing comes too shallowly, with little hitches and sighs, how Sam jumps when Dean pets his thigh that this is Sam's very first time.
He sits Sam on the bed because he remembers how hard it was to stay standing up the first time, when the wet-fucking-hot-heat made him feel like he would die or break into a million pieces. Dean undresses Sam tenderly, pressing his mouth gently to each part of Sam's body as it's exposed because he remembers the debilitating fuck-don't-look-at-me self-consciousness of impersonal sexual encounters. And Dean does not want Sam to ever experience that.
Dean strokes his thighs, touches his balls and perineum for the first time, tender and slow. Sam shakes and Dean moves slower. He knows Sam is terrified as much as he is turned on, but he's always known how to make Sam feel safe. It's his job.
Dean presses soft kisses to Sam's erection, laps at the overheated skin. When Sam grabs the duvet, white-knuckled, Dean reaches out and guides his brother's hands to his hair and shoulders.
"Just hold onto me, Sammy," he whispers.
Then he takes Sam in, working him slow and deep. He holds one of Sam's hands with the hand that's not cradling Sam's balls. Sam squeezes his hand hard, other hand tangled in Dean's hair, and Dean smiles. He doesn't need the little whimpering noises Sam is making, nice as they are, to know that Sam feels overwhelmed, as if every inch of his skin is on fire. It radiates off him in waves of oh-fuck-Dean-can't-I-can't-anymore-please-Dean-ple ase-so-good.
So Dean's gentle, but he doesn't relent until Sam comes, sobbing and arching off the bed. Dean holds Sam in his mouth, puts his arms around him to hold him still and keep him warm, as he goes flaccid. And when Sam comes down, crying softly and clutching Dean's shoulders, Dean lets Sam slip out of his mouth. Dean cleans him up, careful because he remembers the ow-fuck-dammit-boneless tenderness after his first time.
Then Dean lays next to Sam, lets Sam rub him off, just inhaling the smell of this Sam, this new Sam with his post-first-time-newness smell. After, Dean wraps Sam up and whispers everything he knows but never says into Sam hair as he falls asleep in Dean's arms.