(Not going to do this all in yellow as the post's kinda long)
Well no one knows for sure why gays are gay. As a gay man, I can tell you it's NOT a choice, anymore than you being heterosexual was a "choice" for you.
In fact, far being from a choice, most gay men struggle with their homosexuality for a period of time, and try to deny it to even themselves, before "coming out of the closet".
Let me tell you why the right wing groups would like you to BELIEVE that it's a choice: If they're able to convince the public as a whole that homosexuality is just a "lifestyle decision", they have a strong arguement against providing protections and civil rights benefits to them. No marrage, no civil unions, no nondiscrimination clauses.
If, on the other hand, the public learns the truth, that homosexuality ISN'T simply a life style choice, but a true uncontrollable, unchangeable part of someone's very being, their arguments against providing equal protections become very clear for what they really are: bigotry and hate.
If you're serious about learning more about the topic, the Boston Globe did a rather extensive piece a while back, I dug it up for you:
http://www.boston.com/news/globe/mag...es_people_gay/