appears to seriously overestimate the degree to which a person has control over their sexual orientation.

I don't think control as such is the issue, though; at least, that's not how I read Multiheaded's comment. It seems at least plausible that human sexuality is at least somewhat malleable to cultural inputs: even if no one consciously and explicitly says, "I hereby choose to be gay," it could very well be that a gay-friendly culture results in more people developing non-straight orientations.

If nothing else, there are incentive effects: even if sexual orientation is fixed from birth, people's behavior is regulated by cultural norms. Thus, we should expect that greater tolerance of homosexuality will lead to more homosexual behavior, as gays and people who are only marginally non-straight feel more free to act on their desires. For example, an innately bisexual person might engage entirely in heterosexual behavior in a society where homosexuality was heavily stigmatized, but engage in more homosexual behavior once the stigma is lifted.

Thus, conservatives who fear that greater tolerance of homosexuality will lead to more homosexual behavior are probably correct on this one strictly factual point, although I would expect the magnitude of the effect to be rather modest.