A lot of the musings I've seen in this direction have been along the lines of (1) some people are bisexual and thus would occasionally reproduce and (2) if se

A lot of the musings I've seen in this direction have been along the lines of (1) some people are bisexual and thus would occasionally reproduce and (2) if sexuality promotes "togetherness" and "group cohesion", then having "our gay uncle who 'has connections' and is always willing to help out the family group" is still beneficial for the family unit and thus a gay gene would not be selected against within that family lineage. Probably not a majority of the population, but there's no reason for such a "gay gene" (if it were to exist) to be driven to zero.