It makes great sense to me why there are people who feel visceral disgust at the prospect of sexual activity with their siblings, and that there are men who feel visceral disgust at the prospect of sexual activity with men.

The first is clear to me, but the second isn't. Why would homosexual male sex be a bad thing, as long as it didn't cause men not to seek out women as well?

Of course some resources would be spent on it instead of on mating, but humans have a lot more sex in general than necessary for procreation, and many other 'unnecessary' social activities like games. The usual reasons given (e.g. bonding) also make sense between male pairs. Such sex could (counterfactually) also relieve some sexual tension without inviting jealousy, since another man might provide variety and quick simulation but not replace the long-term woman partner.

This seems to apply even more strongly to women, whose fitness doesn't benefit from promiscuous heterosexual sex like male fitness does.

We can't accept any explanations that would make, say, half of men gay, because that doesn't fit with the facts!

We also need to understand why those explanations are in fact wrong, otherwise we're risking retrofitting explanations to the data by choosing explanations without fully understanding what makes them right.