There was no choice involved, and that's the point.
Neither one of (I assume) had to sit one day and ask ourselves "am I sexually attracted to people of the opposite sex, or am I sexually attracted to people of the opposite sex?"
I did not, and I am assuming that neither did you.
If homosexuality is a choice, then I could choose to be attracted to men...I can't.
If I can't make the choice, then I must not have a choice, and if I don't have the choice of finding men attractive, then the whole "homosexuality is a choice" theory is crap.
On the question of choice, it must be noted that all sex but rape is voluntary and thus every sexual act involves a conscious choice. A person's inclination toward a form of sexual conduct may not, for any number of reasons, be consciously chosen, but the mere existence of desire does not justify the act. To accept otherwise would be to validate adultery and pedophilia.
Nevertheless, there is plenty of evidence that homosexuality is not innate. There is a very considerable body of testimony from tens of thousands of men and women who once lived as homosexuals. These ex-"gays" have renounced their former lifestyles and many have become heterosexual in self-identification and desire.