I could see a time in prehistory where either homosexuality or some other infertility mechanism in small percentages of the populations would be advantageous in large enough groups. Your uncles and aunts help with all the hunting and gathering, but their efforts would concentrate towards you instead of being diluted among your cousins, giving a small number of individuals a great chance at survival instead of a large group a smaller chance of survival. There seems to be a correlation between the number of children a couple has and the likelihood that the later siblings are homosexual, which would support the theory. It would be interesting if you could control for the effect of homophobia in western cultures. Perhaps the biological advantage of homosexuals uncles and aunts is more pronounced in cultures that do not socially ostracize them from their original families.
Homosexuality is also
displayed differently in different species. There are other animals that have observed homosexual individuals in the population and there are interesting theories about the purpose that it might serve as part of a holistic approach to species survival. Human homosexuality genes are probably more helpfully considered at the species level rather than the individual level.