Dean hums quietly to himself as he closes the motel door behind him. He's just come back from a food run. Dad's out, and Sammy's got a girl over.
Dean laughs to himself. His little brother - Dean pointedly ignores the fact that Sam's got a good six inches on him now - has a nasty habit of falling for girls; he doesn't seem to consider the consequences of the job. Dean knows this one - Millie? Maddie? - won't last any longer than the others, but he can't help feeling a little uneasy every time he hears her name. Sammy's only fifteen, after all, and hasn't figured out yet that his hormones mean lust, not love.
Dean hears a muffled moan from up the hall. His first panicked thought - something's happened to Sammy! - is quickly replaced by Sammy, you dirty little bastard! as he recognises the moan for what it was. He's met the girl, and he'd bet his pistol that she's not a virgin... but as far as he's aware, it's little Sammy's first time with anyone other than his left hand. He chuckles to himself as he heads for his own room - and then freezes as he hears his brother's low, guttural moan.
Mandy, that was it, thinks a very small part of Dean's brain. A small part, because the majority of his attention is on his cock, which seems to be semi-hard inside his jeans. Dean's not really sure why, until he hears Sam moan softly again, and suddenly there's no semi about it; he's straining against the faded denim.
Without really thinking about it, he moves closer to Sam's door, and peers through the gap in the doorframe. Sam's spread out on his back, gripping the headboard with one hand and the girl's hip with the other as she rides him. Dean is fleetingly disappointed that she's facing away from him, but then decides that the view is just fine, thank you as he watches Sammy's facial expression change with each thrust.
Dean doesn't even realise that he's undone his jeans until he starts to stroke himself - not too hard, because that's a mess he really doesn't want to explain to his father, but he's transfixed by the scene before him, and he needs some relief.
Suddenly, Sam's face contorts, and Dean knows what's coming: sure enough, he hears his brother choke out "Fuck!" and clutch the girl to him. Dean flees softly to his own room, where it barely takes five strokes before he's afforded the same relief.
Dean's sure he's gotten away with it, until the note's pushed under his door about half an hour later:
Enjoy the show, big brother? ;)