"Is your little bottom sore?"
FLASHBACK
..."Arthur, Arthur, ARTHUR..." Merlin chants as Arthur drives into his body forcefully, driving deep enough into him to make him see stars.
He can't remember who started it, (Arthur had stood up and Merlin's mouth had gone dry) or who continued it (next thing Arthur knew he was against a tree and being taken apart by Merlin's tongue.)
He is near incoherent with pleasure and right now he just wants Arthur to keep doing this as long as possible. His hands grip Arthur's perfect ass and he knows he's leaving the marks of his nails in the flesh.
They are trying (and failing) to be quiet. They did go a ways away from the others but Arthur had smirked in that way of his as they'd walked out here, hands entwined, and Merlin knew he'd be screaming by the end of it (not that he minded.)
"Merliiiin..." Arthur wails, just on the edge of madness. He thrusts one more time and has the foresight to silence Merlin as he rises up beneath him, back arched in ecstasy, and comes, screaming into Arthur's mouth. Arthur comes next and muffles his shout in Merlin's bony shoulder, then collapses on top of him. Merlin doesn't mind this either. He strokes Arthur's blonde head until he comes to enough to move...
PRESENT MOMENT
"It's not as fat as yours," Merlin says, but Arthur knows what he really means.