Two points:

• The best way to justify a genetic basis for homosexuality is to look for its benefit not in the homosexual individual, but in the benefit their homosexual phenotype brings to their community of genetic relatives. For example, if a community has a small percentage of homosexual individuals, then those individuals are working to add food to the community, while not at the same time adding more mouths to feed through reproduction. (This has been called the "gay uncle" hypothesis.)

• Somewhat related... one of the few known predictors of homosexuality is called the "fraternal birth order effect". Basically, for genetically male individuals, the more older male siblings they have, the more likely they are to become homosexual. It's somewhat reasonable to extrapolate that having a large number of siblings could somehow function as an environmental signal that checks population growth.