Do same-sex marriage opponents need the choice thesis, and do same-sex marriage supporters need to deny it?

By the choice thesis I mean the claim that sexual orientation is chosen, that someone can make a choice which results in their being gay or not gay.

A version of the no-choice thesis would be the genetic thesis. I remember finding out that a pair of identical twins my family knows have one straight twin and one. My first reaction was that this closed the case against the gay gene thesis, but biologists have made the argument that a genetic disposition to homosexuality might be the result of epigenetics.

I am going to argue that the answer is no to both questions, and that the choice thesis is probably true in some cases but not in others, unless you define everyone who can make a choice as bisexual.

Same-sex marriage opponents Sean McDowell and John Stonestreet write:

Note that we did not say that homosexual inclinations are necessarily a sin. Unfortunately, some Christians, often our of deep concern for those struggling with same-sex attraction, promise that Christ will change one's orientation from gay to straight. He might. Many, like Rosario Butterfield, have experienced a change in their sexual orientation. Others, like Wesley Hill, did not. We should neither make false promises about change in orientation nor ignore its possibility. We must tell the truth. 

On the other hand, too many Christians conclude that God must be OK with homosexual behaviors or else He would take those inclinations away. This denies the historic, consistent witness of the Church to the testimony of Scripture. Any sexual activity outside the given norms of marriage is sin. We must tell the truth. 

The book by Wesley Hill that I linked to was a book entitled. Spiritual Friendship: Finding Love in the Church as a Celibate Gay Christian. In other words, McDowell and Stonestreet hold that there are unchangeably gay people, but that they are morally obligated to be celibate. Now some might be thinking that it is an awfully cruel God who would demand such a thing, but if you are congenitally attracted to little boys and only little boys, it seems to me the only reasonable thing to conclude would have to be that there is no moral way to have a sex life. If you are married and your wife has contracted a health condition (and there are conditions like this) that makes intercourse always painful, does this give you the moral right to find other partners?

I suppose it might be possible to deny the choice thesis by arguing that everyone who goes from gay relationships to straight ones were really bisexual. But this would, of course, suggest that the population of bisexuals is considerably greater than most of us would have initially thought. If the case in defense of gay relationships depends on the "no choice" thesis, then not only could traditionalists argue that those in that class ought to be celibate, but it also opens a "weak" version of
traditionalism which suggests that if you have no choice, then gay might be OK, but if you have a choice, you ought to choose hetero. There are large portions of the LGBT community who are not going to like this, especially the Bs.

On the other side, defenders of gay rights are moving away from the "born this way" argument, claiming that if one is really able to defend homosexuality, then one ought also to be able to defend it as a viable and morally acceptable choice. See here.