You could argue it as potentially a natural ingrained form of population control. If a segment of the species is only attracted to the same sex then they aren't going to be reproducing, which would therfore mean a portion of the population is stagnant and therefore not creating more mouths to feed. If homosexuality was partially genetic, then it would be for a reason, as the process of evolution is complex and ever changing. Therefore, I bluntly state that your statement is incorrect.