Because humans have a wide variety of natural emotions, desires and thoughts, it is worthless to ask if a particular sexual attraction is a matter of choice or not. Who chooses what emotions and desires they will have? The better question is: of all the emotions and desires a person might have, which are moral to entertain and which are immoral and must be rejected?
Asking if having gay feelings is a choice dodges that question entirely. We are told, if a person feels something, it must be natural. What if a person feels he should murder or abuse someone else? A person with normal thoughts knows to reject any hatred that rises to the level of wanting to hurt or kill someone else. Why is it any different with sexual feelings? I think only because too many people want to excuse the moral implications of sexuality.
But I think it is deeper than that. God clearly tells humanity what His rules of sexual morality are. He clearly states in several places such as Revelation 22:15 that those who are sexually immoral cannot inherit the Kingdom of God. So, I think that some people use sexual expression as a way of rejecting God and His moral laws.
Because they have not read their bible? Only those that have accepted the word of God understand the "moral implications of sexuality?"