Rather than be seen as failures and freaks in a normal society, they choose to become part of a sub-culture that celebrates freaks.
I tend to agree with this rather than your argument, Laissez-faire, because in essence, yours is saying "Choosing to be gay is just as innocent a choice as choosing a political affiliation, or religious belief. In other words, they (the gays) are simply choosing to express OUTSIDE what they 'are' INSIDE".
IOW, that argument really doesn't negate the concept of "born gay" at all. In fact, in a way, it re-enforces it.
I prefer what JustRight said. Basically, the reasons homosexuals DO choose what they do is because misery loves company. And the more we normalize this sick behavior, the less that truism becomes apparent; the more we DO look like "homophobes", as we continue to fight against, as you put it Laissez-faire: An outward expression of what someone is inside.
So the fight really shouldn't be about arguing WHY homos choose their lifestyle, it should be about the lifestyle ITSELF, demonstrating how destructive it is. Once you do that effectively then the rest becomes easy. A person can "choose" to be anything, but as long as society deems it inappropriate then the choice is clearly just that, a choice. There's always going to be sick people in a society, IOW. What we choose to do with them, as a group, is what really defines (or denies, as in the case of homosexuality today) their sickness.
Once the battle shifts from the effects of the choice to the choice itself though, then it's basically over, the battle is lost. Because who, in a free society, is going to tell someone, "You can't make choices for yourself!" Indeed that's the subtle danger of attacking the choice, and not the lifestyle of homosexuality. You're giving them (homosexuals) automatic martyr status. A perfect catch-22 in other words, if you really think about it.
What has happened is that the failures and freaks and just plain moral degenerates have successfully changed society so that they are no longer considered failures, freaks and moral degenerates. Their behavior and attitudes are now acceptable, even the norm.