Derek Morgan has a habit of walking around his house naked. Spencer Reid has never complained.

He knows he's staring. He can't help it, and he can't even pretend it's pure academic fascination, although recalling the knowledge that the gluteus maximus muscles in humans are much larger in comparison to other primates because of their role in maintaining the erect posture of the torso comes as easily to him as breathing.

He's just wondering whether his fascination could be considered pygophilia – buttock fetishism – or not, when Morgan turns around and he's suddenly staring at crotch. He looks up, and the other agent is smiling knowingly.

"Staring again, pretty boy?"

"Yes." He doesn't lie, there's no point, and he doesn't want to.

Morgan sidles over to the bed where Reid is perched and climbs onto it, laying down on his front and resting his head on his folded arms. It's quite clearly an invitation, one Reid doesn't deny. He reaches out and grabs the man's cheeks, groping them as he bites his lip. It's a perfect thing, really, Derek's ass; smooth chocolate brown, large and firm and strong, but fleshy enough that when he walks there's a little jiggling motion that drives Spencer completely wild. He taps it with the fingers of both hands to cause that now, watching in fascination as the flesh moves and then stills. He hears Morgan let out a breath of a laugh, and he can't help himself so he leans down and peppers kisses over Morgan's warm flesh.

"You're an ass-man, Spencer Reid." Morgan murmurs. Reid bares his teeth and scrapes them on the other's dark flesh, relishing the long low moan that creates. He flicks out his tongue too, to counter the stimulation from his teeth; he bites down on Morgan's sweet flesh, hard enough to leave an impression of teeth and then laps at the mark, soothing the sting and tasting the smooth heady favour of Morgan's flesh.

He doesn't know quite what it is about Morgan's backside that gets him in such a state; it's strange, because while he considers the sexual attraction he's experienced in his life normal, nothing has ever affected him quite like this. There are days when Morgan wears pants that are really tight on his rear and Reid actively has to avoid looking at it, or he'll get a badly timed erection. It hasn't always worked; he's had knowing looks from Garcia and Prentiss, and even a quirked eyebrow from Hotch that was mortifying.

His current erection isn't mistimed, however, as Morgan shifts a little, breath short and shallow as he lifts his rear into the contact Reid is giving him.

"You hard, baby?" Derek asks, knowing the answer, but wanting to hear the other say it.

"Yes." Spencer has been hard the entire time Morgan's been walking around naked.

"You wanna cum on my ass?" he purrs.

"Yes..." Reid moans, squeezing Morgan's backside and biting down again. He wastes no time, scooting up Morgan's body, knees either side of his lover's thighs so he can put his cock in the cleft between his cheeks, and rocks his hips back and forth. He knows he'd not going to last long, it's been a busy week with little time for sex, and then Morgan starts to clench and unclench his rear muscles, making the flesh of his backside flex. Before long he has to stop sliding his length in the dip between the globes and takes his cock in hand, bracing his other on one of Morgan's ass cheeks as he strokes himself.

With a throaty moan he explodes, a streak of pearly fluid streaking across Morgan's smooth brown left buttock, then another line of cum, and another. He moves his aim then, stroking furiously as four smaller spurts paint his lover's right buttock. There's some quip about his hand-eye coordination forming in the back of his mind, but he ignores it as he rests back on Morgan's legs, still lazily stroking his cock and marvelling at the pattern of white liquid over the other's skin. He's sure Morgan's dark skin makes his semen look whiter, and the whole thing looks like some obscene and deliciously iced cake; which feels like a ridiculous analogy, so he keeps it to himself.

"Derek..." he murmurs.

"Such an ass-man..." he hears his lover whisper, knowing it's true, and knowing when he's caught his breath he'll go down and lick up every trace he's left over such perfect skin, and he won't stop until Morgan is lying panting and spent beneath him.