I have long been dissatisfied with the attempts I’ve seen to explain why evolution hasn’t made homosexuality rare.
A new paper in PLoS ONE: Sexually Antagonistic Selection in Human Male Homosexuality presents a model that seems adequate.
The main idea (from this summary) is that
a heightened sexual response to men could make women more likely to pass on their genes, while making men possessing the trait less likely to do so.
The paper notes this evidence to support it:
homosexuals’ mothers are more fecund than mothers of heterosexuals. Further female fecundity asymmetries include a higher fecundity of maternal vs. paternal aunts of homosexuals
This model provides hints about why homosexuality is not rare among other species.
My main reservation is that the model leads me to expect bisexuality to be somewhat more likely than homosexuality.