Let's say it is genetic. It could be a mutation that's survive because it acts to curb the population to a degree.
Might be something to that. A few years ago they had a study that showed that the more sons a woman has, the more likely the next one is to be gay.
Of course, if it were genetic it could also be a recessive gene, and recessive genes can't be bred out. Could also come from a combination of genes that are advantageous by themselves but disadvantageous in combination with each other.