And traditional behavior gives us an imperfect window into the economics of the past, which is what's under discussion when we talk about historical selective fitness.

Traditional behavior is so widely varied, though, that it's difficult to draw any conclusions. Some traditional societies practiced polyandry, others, polygamy, and still others, levirate marriage, and avunculism, and so forth. Some traditional societies were accepting of homosexuality and even transgenderism. You say that cultures that prohibit childbirth die out, but many diverse cultures have a thriving tradition of monasticism (which is even worse for reproductive fitness than homosexuality!)

If gay sons were helpful enough with nephews and nieces that it was as if they had had their own children, it seems to me they would be welcomed and lauded as examples of loving selflessness.

Would they? "Gay" is a recent category; traditional societies did not attempt to classify humans in that way and it only became popular when religious authorities attempted to criminalize it and early psychologists attempted to medicalize it. Men were not "gay" or "homosexual", they were more or less inclined towards other men.