I too also have to agree that it is not a choice. There has actually been a lot of research put into this concept in Canada, and the results proved to be quite interesting. What they found was that they could determine the sexual orientation with an amazing accuracy based on scientific lab tests. Simply put, there appears to be a co-relation between sexual orientation and the hormone levels that the developing fetus is exposed to. When a male fetus is exposed to a higher proportion of Estrogen than to Testosterone, the results seemed to be more favourable to a gay child. By the same token, a male fetus exposed to more Testosterone than Estrogen seemed to prove to turn out to be "straight".
The opposite holds true for females. More estrogen = heterosexual, and more testosterone = lesbian. Of course, this is not an exact science and there are a few other factors and interesting observations to take into account. The other biggest one being that neurological studies have also been done for curiosity sake and seem to suggest that the hard wiring of a gay man's brain seems to be more closely related and wired to that of a heterosexual female. It's quite an interesting concept actually.