Abel tenses up in surprise at the kiss, but he doesn't resist it. He's never been kissed before, and he certainly didn't expect it to be like this. He can't precisely object, though.

Resting his hands against Cain's bare chest, which is a very nice bare chest and unquestionably the nicest bare chest he's ever had the privilege of touching, Abel tentatively returns the kiss, or at least tries to keep up with it.