How to deal with homophobic friends

In simplest terms: Tear his arguments apart.

Ask him when he chose to be straight. Point out that homosexual behavior has been observed in various animal species and ask him if they 'chose' this. Point out the number of people who suffer extreme emotional distress at being non-straight or who are rejected by friends and family and community and ask him why anyone would 'choose' that. Etc. etc.

You could also point out that sexual orientation boils down to a matter of taste. Does he dislike people because they like food/music/cars he doesn't?

You will probably need to do some research to get your arguments and counter-arguments well formulated, but that's certainly doable.

Coming at this from another direction, even if being gay were a choice, it hurts no one (it's not our problem if thinking about it makes him uncomfortable), so why should he care? Religion is definitely a matter of choice and he's presumably not in favor of persecuting religious people.

Hope this helps,

Todd