Why would homosexual male sex be a bad thing, as long as it didn't cause men not to seek out women as well?

Primarily, that; secondarily, disease risk. It seems to me that there are many men who put up with women only for the sex, and if they could get that satisfaction elsewhere, they would.

The usual reasons given (e.g. bonding) also make sense between male pairs.

It looks like a number of ancient societies had sanctioned male-male sexual relationships, often but not always of the 'old mentor / young protege' variety. But it's hard for us to tell how common those were (specifically, how many of those partnerships were actually sexual, instead of just knowing that some were).

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

Interestingly, female bisexuality seem much more common than male bisexuality, and also considerably more fluid.