As far as I know, homosexuality is a complex issue which involves many factors. A couple of years ago or so, I read about a study that found out that homosexuality in men was linked to a genetic component which increases fertility in women. So this genetic component makes women more likely to become pregnant (which is why it's evolved), but also factors in homosexuality.
There are also many studies pointing towards the importance of the fetal environment. It's now "famously" known that the more older brothers you have, the higher chance you have of being gay. I can't remember the specifics behind this, but it has something to do with male fetuses triggering an immune reaction in their mothers which becomes stronger with each successive child.
This bit about homosexuality on 60 minutes is getting a little old, and I'm sure I posted it in another thread, but it is really interesting.
You guys should watch it if you haven't
