What suggests that homosexuals are getting less sex than heterosexuals in the first place?

Genius being easier to claim in retrospect, I think the real claim is that until recent decades, there were plenty of nearly celibate homosexuals (for lack of public opportunities to seek out others, or from internalized stigmas).

Obvious thing to check is the contribution to science and art from other known celibates; plenty more examples (including Erdös) leap to mind.