Ironically its far more likely that those who say it's a choice are the ones who are gay or bi but in denial about it. Because they look at it like "Well, I understand the appeal - I could easily go for a member of the same sex because obviously that's quite a tempting option that everyone would just love if they stopped restraining themselves - but I choose not to. If I can chose not to, anyone else could too, because everyone is just like me, and I 'know' I'm perfectly typical and straight." People who are actually 100% straight understand entirely that they couldn't really make themselves enjoy homosexual sex by choice, so they "get it" that people who are 100% homosexual probably can't choose either.
The "it's a choice" crowd are basically envisioning a world where everybody is bi, which is ironic because they'd claim they would hate that kind of world and not promote it at all.
And that's also why I think it's a bit of a mistake to build the argument on why there's nothing wrong with homosexuality on the foundation that they can't help it. Sure, they can't help it, but that's NOT why it's okay. I mean, for those people who *could* swing both ways, does it stop being okay when THEY do it because they DO have a choice? Of course not, but hanging the moral argument on "can't help it" sort of implies that. The "where's the victim? No victim, no crime" argument is far, far better because it makes it equally okay for homosexual people and bi people.
Also, as the science of the brain gets more and more advanced, there may come a day where it would actually be possible to alter a person's sexual preference artificially. If society still sees it as a problem that's only being allowed because "they can't help it", then once there *is* a medically proven "cure" for this not-a-problem problem (unlike the current crop of bullcrap "therapies" which are basically just abusive browbeating and authoritarian control till someone says what you want), society may swing back to "well, we have a 'cure' now, so we can 'fix' this 'problem'." It could be like the days when Alan Turing was made to take a series of hormones as if that would "cure" his homosexuality. The "cure" doesn't work, but that's NOT the reason it was wrong for the government to do that to him. It was wrong because he wasn't hurting anybody. If they did have a "cure" that worked that still wouldn't have made it okay for the government to force everyone to become straight.