The first time John had kissed Sherlock's neck, that impossible white, swan's neck, Sherlock had whispered, "No marks, please, no marks." It was a request that John was careful to honor, instead kissing scarlet circles onto Sherlock's shoulder, on his chest well beneath the third button of his shirt, and anywhere else that wouldn't be seen and commented upon. God knows it was sometimes hard to remember, when Sherlock's throat smelled of fresh cotton and tasted like sweet cream. Or, even worse, tasted of sweat and burned with the flush of Sherlock's arousal. And Sherlock liked John's mouth on his neck. He would tilt his head away to expose more of it to John's questing tongue. But John would have to content himself with closed-lipped, delicate kisses and licks along the tendons until he was tasting his own saliva. He could nip along the underside of Sherlock's jaw, or swirl circles in that delicate ear, but never bite or suckle the way he wanted.
But Sherlock never said anything about not marking his bum. And it really was too tempting, as white as every other centimeter of skin on his thin but rangy frame, but nicely defined with a pleasing roundness. It was even more fantastic when it was moist with the sweat from their exertions, presented like a perfect Valentine's Day pillow for John's head to rest upon as Sherlock lay face down recovering.
John twirled his tongue in each of the little dimples on Sherlock's buttocks, lapping away the salty taste. He opened his mouth, latched onto one plush cheek and let himself enjoy the sensation of pliant skin in his lips, so different from the hard skin covered bone of the rest of Sherlock's body. There was a tiny pop at the release of suction when he finally pulled his mouth away. The dark red mark against Sherlock's skin was like a cherry in whipped cream.
"John," came a lazy rumble from further up the bed, "would you rather mark my arse or fuck it?"
There really was only one answer to that. A matching mark on the other cheek would have to wait.