One discussion that came up often when Colorado's Amendment 2--the anti-gay rights amendment--was up for consideration in 1992 was the debate over whether people "choose" to be gay. One still hears echoes of that debate (although it seems to me less salient than it was in the '90s) in respect to the prospect of same sex marriage laws. The debate is a false one that does not represent the real agenda of either side of the issue.
The debate goes like this: pro-gay rights advocates insist that no one "chooses" to be gay while anti-gay rights advocates insist that people "choose" to be gay. A variant of the pro-gay rights argument is that no one would choose to be gay because of the stigma, etc., attached to being gay. A variant of the anti-gay rights argument is the notion that homosexuality can be "cured."
Now, let's assume that incontrovertible proof emerged that people are, indeed, born gay. That being gay is an immutable trait just as skin color or biological sex is (I'll bracket the obvious counter-examples to immutability, such as sex-change operations). Let's also assume that anti-gay righters accepted this evidence as valid and conclusive (yes, I am positing a hypothetical). That doesn't answer their objection that homosexuality is immoral; that only underscores the degree to which preference is unavoidable.
Now, let's assume that incontrovertible proof emerges that people do, at some point, make choices that pre-dispose them to "become" gay, or let's assume that a discovery is made about a pill that can be taken one time to "cure" homosexuality. Such a proof would not answer their objection that discrimination is unfair and unjust.
For the record, while, as a speculative and philosophical proposition, I have my doubts about the assertion that absolutely no choice or act of will affects a person's orientation, I believe that if someone can find happiness with a person of the same sex, then homosexuality is a positive good. Homologously, I believe the state has no business sanctioning discrimination against gays.