There's still plenty of conservative people who believe the being gay is a personal choice, or perhaps something caused by your upbringing/environment, not something you're born with.
Of course, this has as much veracity as the idea that the Earth is flat or that the Sun and planets revolve around it, but there's a lot of people who believe it.
But the whole argument is ridiculous. Maybe there is a choice element to being gay. So what? That is a perfectly valid choice to make. But attacking someone for that is not valid, it is completely perverse.
It seems to me that the purpose of the "born with it" argument is to develop empathy among people who lack understanding. Because it helps explain why people act on their feelings, even if they are not a good idea. We can all relate to the power of lust, love etc. It is an important point to make. But that argument only has relevance to people who find it distasteful in the first place. The argument is a reaction to homophobia.
I agree that it's a valid choice to make, and people should have the freedom to make such choices if they want. However, you said before that "It makes no difference to anyone if homosexuality is innate, or just a set of chosen behaviours." and that's incorrect. It does make a difference, to some people. To those people, they'd oppose it if it were proven to simply be a choice, whereas they'd have more empathy if it were proven to be completely innate: it'd like being mad at a mentally-retarded person for doing something dumb, compared to being mad at or disappointed in a very intelligent person for making a very dumb decision. I'm not defending this mentality (because again, I have no problem with homosexuality and I think people should be free to have whatever physical relationships they want), but I'm explaining it.
Also, remember that a lot of people (particularly religious ones) want to regulate social behavior, even in the bedroom, because they believe this is important for society's survival and prosperity. So in their view, homosexuals should be oppressed (either strongly by the state, or more weakly by social pressure) for the good of society, and to keep their behavior from "spreading". There's not really a way to counter this mentality by people like us because their worldview is so entirely alien to us: we don't believe in divine retribution for "immoral" behavior (a la Sodom & Gomorrah) and they do, and nothing's going to shake their belief.
>There's not really a way to counter this mentality by people like us because their worldview is so entirely alien to us
As you hint at up above, there are non-religious people who believe regulating sexual behavior is important for society. Do you have a way to counter that idea when it's based on a secular worldview which is not so alien to yours?
Liberty versus authoritarianism? I acknowledge such people exist, but I don't pretend to know or understand their full argument of why it should be regulated. At least with the religious people, I can understand why, I just reject the silly premises (that God will smite us like in the story of Sodom). With atheists (presumably Stalinists?), I really don't get it. Besides, we've seen atheistic authoritarian societies before, and they were a complete disaster, not only economically but also as far as having a happy, functioning society. Hint: if you have to have a wall to keep people from running away from your society, and you shoot people who try to climb over the wall, then the fundamentals of your society are not worth preserving. Now it might be obvious that I'm following a fairly utilitarian philosophy here (happy = good), but if you're not religious and don't believe in utilitarianism, what exactly is the goal for your society? Generally, societies which do not value the happiness of their citizens are religious, because they claim that they need to "please God" or act in certain ways to have a better afterlife. Remove the afterlife and deity and what do you have left? Either wanting people to be happy (but still functional; not happy as in drugged-out all the time and unable to keep society functioning), or you're running the society on what makes one person happy and to hell with what everyone else wants, which is basically narcissism (which probably describes North Korea today).
>but I don't pretend to know or understand their full argument of why it should be regulated
One reason might be that they believe that having a culture which is hedonistic toward sex is harmful for society, and they believe that acceptance of homosexual behavior necessarily leads to having such a culture.
>presumably Stalinists?
That is definitely not a reasonable assumption to make.
>but if you're not religious and don't believe in utilitarianism, what exactly is the goal for your society?
It is perfectly possible to be utilitarian, non-religious, and opposed to the acceptance of homosexuality. All it takes is a belief that acceptance of homosexuality is on the whole and over time more harmful for people than intolerance of homosexuality.
Of course, this has as much veracity as the idea that the Earth is flat or that the Sun and planets revolve around it, but there's a lot of people who believe it.