I don’t think the second case can fit homosexuality. If homosexuality-promoting alleles are losing ground to mutations that do better, why haven’t they completely vanished yet? It’s unlikely that going back, homosexuality rates were far higher; that would make prehistorical men very unusual among other mammals.
One possibility is spontaneous mutation. Again you would have to plug in a rate for that and see what the mathematics says.
Another is that the genes involved aren’t mutations, they’re alleles that are losing ground to mutations that do better.
Whether either of these or something else can fit the case of homosexuality I don’t know.
I don’t think the second case can fit homosexuality. If homosexuality-promoting alleles are losing ground to mutations that do better, why haven’t they completely vanished yet? It’s unlikely that going back, homosexuality rates were far higher; that would make prehistorical men very unusual among other mammals.