I think it's a genetic mutation of some kind.
It's certainly not a conscious choice. There are far too many examples of homosexuals living in repressive regimes where they are threatened with prison or death for homosexuality. And since heterosexuals do not recall ever choosing to be heterosexual, the same follows for gays.
Environmental...it could be, partially. But plenty of segregated, patriarchal cultures have still had very low incidences of homosexuality. As far as can be ascertained, single sex schools do not produce more homosexuals than co-ed schools.
That leaves only biology. But of course, no matter what the cause, there is no reason to treat it as anything wrong or inferior to heterosexuality.
(Former) Lead Moderator and (Eternal) VGC Detective 
