Sexual orientation isn't a choice at least for men. Whether it is for women or not, I can't say from my own experiences. But I would never choose to be gay, and the idea of experimenting in that way doesn't appeal to me at all.
As far as nature or nuture:
There is an SRY gene on the Y chromosome that does have a slight prediction value of homosexuality or heterosexuality in men. That does point to genetic factors. After a mother has more than one son, each individual son after that is more likely to be gay than the one previous. They think that may be an environmental factor, but in the womb, not in the actual genes. Matt Ridley devotes considerable discussion to both of these factors in his book Genome: the autobiography of a species in 23 chapters.
Evidence for whether nature or nurture plays a bigger role in preferences for women in general is more sketchy even than that, or at least was the last time I looked.
Rob: That's a non sequiter. To my knowledge there are no large segments of the population that has an overwhelming biological urge to murder and a predisposition of who only an afterthought to the actual act.