He isn't quite laughing, not for lack of merriment, but his lungs are busy helping him to pant, and his muscles are busy settling themselves before their next move. He's grinning anyway and his amusement is plain on his face; he leans up for another quick, playful kiss.

"I did think that you might, for the record. I weighed the possible outcomes -- accidentally doing you a harm versus finding you atop me, naked and grinning -- and decided I favored the latter."

It doesn't mean that he isn't about to retaliate either, for that matter, and he counterattacks, attempting to get the upperhand over Watson, which instead leads to a prolonged battle for the top position. They are too well-matched and not overly interested in coming out the victor for anything to escalate beyond playful tussling.