(1)Putting the ocean of anecdotal evidence against this claim aside, there is no scientific consensus supporting the claim that homosexuality is a choice in the vast majority of cases(2) - quite the opposite, no credible American scientific organization would support that claim.(3) Because sexuality is more than brain chemistry, scientific studies will never tell us all we want to know about ourselves,(4) but the evidence that homosexuality is not a choice in the vast majority of cases is consistent and overwhelming.(5)
1. This is a core part of the argument that homosexuality is sin. Sin often involves our volition. We will to do something wrong. In some cases, it is possible for something to be sin and not a choice, but we can deal with that later in more detail. If homosexuality is not a choice as, is becoming increasingly accepted even among conservatives, then an element of moral guilt is removed. This is partly why opponents of inclusion are so eager to separate between sexual orientation and behavior, but the division between being and action is never clean.
2. By ocean of anecdotal evidence, we mean: go speak with a homosexual person and ask them when they decided to be gay. Or, conversely, think about when you chose to be heterosexual. Of course you never did. Neither did they. In a small number of cases, a person's natural sexual orientation might be corrupted by abuse or other environmental factors, in which case the loving thing to do is to nurture them toward healthy, consensual sexual expression. Here's a pastor saying just this from the recent news. For him, his sexual orientation was no more of a choice than the call of God on his life - in his own words. This is true of LGBTQ folks who want to be ordained this very moment.
3. As for scientific consensus, homosexuality was removed from the DSM in 1973. The American Psychiatric Association, the American Psychological Association, the American Counseling Association, and any number of other scientific organizations take the position that in the vast majority of cases, homosexuality is not a disorder and is not a choice. Numerous long term twin studies have come to the same conclusion. If you argue that homosexuality is a choice in most cases, or even in a large minority of cases, you are doing so apart from the best science available.
4. This chunk of counter-argument is leaving aside ethical or theological considerations for the most part, and simply dealing with the false claim about choice that under-girds so much anti-equality rhetoric.
5. In fact, claims to the contrary are essentially the sole purview of religiously-motivated groups which function outside the world of scientific study and peer review.