I would say your progression breaks down here, where you conflate homosexual orientation (i.e. being gay) with homosexual activity (i.e having gay sex). Any single act of homosexuality is voluntary and not required for the survival of the individual, as is every act of heterosexual intercourse (although I will admit, there have been times when i was sure I would die without some...). But the real question is homosexual orientation, which again, is not required for the survival of the individual. BUT, being heterosexual is also not required for the individual's survival (you may say heterosexuality is necessary for species survival and homosexuality is not, but since one of the explicit questions of this thread is whether or not homosexuality is also good for the species, we can't use that escape hatch). So we have to ask the same question about heterosexuality as you did about homosexuality - choice or mental illness. I flatly reject that my heterosexuality is an illness, and I doubt anyone will argue the point, so we're left with it being voluntary. And here's the problem: I don't recall making that choice. I have always been what I am, and I've always known it. Furthermore, I doubt any straight person on the planet has a recollection of choosing to be straight. So, we have a quandary - heterosexuality is either a choice or a mental illnesses, but we didn't make the choice and we know it's not an illness - so we have to choose option three, that it is a natural healthy state. But perhaps homosexuality is different, perhaps it really is part of your dichotomy - a choice or an illness. I'm sure homosexuals feel just as vehemently as heterosexuals on the illness option, so we'll toss that out, as you did in step 3. Now we're left with choice, which we disposed of on the hetero side of the equation. Here too, we have a simple test: someone has to decide to be gay. So here's an experiment - one straight person has to decide to be gay, even for just a while. They have to wake up in the morning and say "for the next week, I'm a gay man!" They have to look at men the way they'd look at women, and get the same same thrill from a bulging package as they would from a well-filled bikini, and they have to really mean it. Can you do it? I can't, I can barely type "bulging package" without gagging. Can anyone do it? Until I meet someone who can actually make this choice, I reject the idea that homosexuality is a choice, that it is anything but a natural, innate state of being. Is it good for the species or society? I guess that's a question for another day...
Substitute just about any behavior for homosexuality and your statement breaks down. Try, for example, gluttony. Fat kids don't remember making a choice to be fat. Kids with bad tempers don't rember making the choice. Neither are natural or healthy states.