I firmly believe that no sexual feelings are a choice. Why would anyone choose to have far less choice of partners, face prejudice and discrimination (on a regular basis, if you live somewhere that doesn't tolerate "the choice"), even face DEATH penalties if you're outed as homosexual? Why would anyone want that?
Furthermore, there are thousands of species who have homosexual behaviour within them. I've personally met a pair of gay penguins (or maybe they were bi, who knows?). How much more natural can you get?
I think that homosexuality within a species is perfectly normal and is a natural sexual variation, which is a complex combination of both biological and environmental factors. Although I do like the idea of straight, anti-homosexual parents carrying around 'gay genes' that made their child gay, I think it's probably more complicated than that. Just my thoughts, anyway!