Back to the OP...

There could be a genetic mechanism for mothers having homosexual children. It (obviously) wouldn't be a single allele making someone gay, but bear with me for a long-winded explanation...

Imagine in a tribal society, a man marries a woman, moves into his own house, and starts having children with her. Each child has 50% of his genes, and is raised using only his own and his wife's resources. Under this scenario the family manages to get n kids to adulthood.

Now imagine he stayed home with his extended family instead of moving out, and his brother who owns too many pairs of sandals volunteers to help with the kids because it's not like he's busy saving up a brideprice or anything. Each child has roughly 25% of the genes of Uncle Softie, is raised using the resources of three people, and no one had to go buy a brand-new house to shelter them. If under this scenario the odd crowd get 2n+1 children to adulthood, the rose tribesman will be more successful genetically than if he hadn't been a friend of Dorothy, and he still gets to be the confidant of all the girls around the village.

You might notice that this strategy is only viable if you have siblings, and a family with one son checking out the ladies and another three checking out fashion magazines is going to get diminishing returns. But interestingly, a trend among humans has been noticed where homosexual men are more likely to have several older brothers, and when you apply some serious statistics to this trend, it turns out that the more sons a woman has, the more likely her next one is going to be a tad bit lavender. Note that if there really is a genetic cause to this, it would be a trait for which the sons are carriers, and it's only actually expressed in mothers while they have buns in their ovens.

So in the end, no simple gay gene, but we have evidence something inheritable can cause homosexuality.