This seems like just playing with words to me. Two men having sex with each other is a homosexual act, simply by definition.
Are all homosexual acts "gay"? Not if "gay" involves something more than homosexuality, which it might. Perhaps it requires identity as well, or some kind of self-consciousness. But if "gay" is taken as essentially such a kind of identity, not just behavior pattern or a disposition of one's passions, then there would be no sense in discussing gay sex beyond what a gay person sees as an expression of his/her identity. It would also follow that plenty of same sex sexual acts are homosexual, but not gay. In that case, "homosexual" would really be what describes what is intrinsic to the kind of act, not "gay". And if this were so, then there would still be no difference between what "straight" men do when they have sex with other men.
Every man who has sex with another man is doing the same sorts of things, and is doing them because other men have some sort of sexual appeal for him. In that respect, some instances of homosexual sex being "gay" and others not doesn't mean anything coherent or relevant. The underlying problem seems to be equivocating on the meaning of "gay", and not grounding the term down in a specific, consistent sense.