Derek likes it when it hurts, that soft, soft mouth with its sharp, sharp teeth, and the sharpness of Stiles's eyes, of his smile, as he smudges his lips along Derek's cock, right along where his teeth had brushed it, leaving faint lines of fire in their wake. And he does it all so lightly, so cruelly, that it hits Derek's system like a shot of morphine, the comfort of that mouth after that flicker of hurt, that wet-smooth tongue after the threat of injury.
"You're a sick bastard, Derek," slurs Stiles afterward, almost lazily, still rubbing his downy cheek against Derek's dick, cleaning it with slow, tingling licks, his eyes dark and at half-mast. He'd come, as well, and one of his hands is still cupping himself, stroking the slick mess down there in time with the same dreamy rhythm he's using on Derek.
"Speak for yourself," Derek rasps eventually, his hand still clenched hard on the back of Stiles's neck, possibly bruising it, but Stiles likes that, too. That edge of -
Not outright pain, but something gentler, deeper, more insidious, an ink-stain spreading beneath the skin.
They mark each other like this. Although Derek's marks are too quick to fade. Perhaps that is why Stiles enjoys hurting him, a little, giving him this gift, this moment, this illusion of vulnerability.
When he tugs Stiles up to kiss him, it's only when Stiles is done cleaning Derek to his satisfaction, sleepy as a cat, and the kiss they share is sleepy, too, gamey with come, salt-sticky and strangely sweet.