There is, apparently, a study showing the likelyhood of a male being homosexual being greater if he has a larger number of male siblings. There is some effect on the mother of previous male offspring that affects the later males. (did I use effect and affect right?).
There are obviously a lot of things we don't know about ourselves. It will be interesting to see what we learn over the next couple of decades.
Something that hasn't been noted in this discusion is that gay or not doesn't seem to be black and white. There are those who straddle the line. Just as physically there are those who aren't clearly either sex. It might be a continuum from one side to the other.