<< Sigh. It is NOT genetic. No one has proven that. And if you still believe it, find the study that proves it. >>
Since reproduction is crucial to the survival of a species, large numbers of homosexual individuals would be disadvantageous. This desire to reproduce is therefore hard-wired and very hard to control. Since homosexuality means that an individual will not reproduce, it means that something has triggered a certain reaction in the brains which somehow disables or cripples this desire to reproduce. Because it's so hard to control or get rid of hard-wired responses etc., it seems very unlikely that homosexuality is just caused by the individual choosing for it.
Some studies with monkeys showed that when there were very few females available for reproduction, the lower ranked males started to show signs of homosexual behaviour. It might very well that this is the case with Humans as well.
