There is a possible link between increased female fecundity and an epigenetic influence contributing to a higher chance of homosexual male offspring. This has two advantages. The first is that higher fecundity means a greater chance of more offspring. The second is that gay sons are less likely to reproduce themselves, but can still contribute to the strength of the family and the survival of subsequent generations.
This only works with group selection. Which is largely dismissed. We´re not ants. Genetic traits incapable of being passed on are worthless. Evolutionarily speaking. Yes, it is interesting that the more older siblings you have the more likely you´re gay. But I have a hard time seeing how this, in any way, could be beneficial to anybody. It´s just placing more eggs in fewer baskets. That´s not how to make a species survive.
The trait is passed on. Hypothetically, it is passed on matrilineally.
It not just placing more eggs in fewer baskets. Generation one produces a higher number of offspring, though perhaps one male of generation two is homosexual and unlikely to reproduce. Even so, the number of reproducing offspring in generation two may be higher than or equal to that of generation one females with lower fecundity. Generation two families with gay siblings may, hypothetically, have the support of those siblings in increasing the survival rate of their own offspring in generation three.
The relationship between increased fecundity and homosexual offspring has been studied to some extent and there is some supporting evidence. The bit about gay offspring helping with subsequent generations is simply stealing from the gay uncle hypothesis, which is not all that important here as the frequency of gay offspring in this scenario could be rather low. Whether this proves to be the right path (or one of them) toward understanding the biology of homosexuality or not remains to be seen, but last I checked it remained plausible with what we know so far.