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Consider the following logic and comment if you like:

Posit: Exclusively homosexual behavior prevents reproduction by its practitioners (without extraordinary medical intervention, e.g., artificial insemination, etc.).
Posit: Innate physical conditions that cause sterility or death prior to puberty (i.e., prevent reproduction) appear in the general population at the rate of a fraction of a percent per condition (morbid recessive gene or deadly mutation rates, etc.).
Conclusion: Exclusively homosexual behavior as an innate behavior cannot exist in more than a fraction of a percent of the general population.

Posit: Any human activity not required for the immediate survival of the individual (breathing, eating, etc.) is either voluntary or the result of a mental illness.
Posit: Homosexual activity is not required for the survival of the individual.
Conclusion: Homosexual activity is either voluntary or the result of a mental illness.

Posit: Adult homosexual activity is either voluntary or the result of a mental illness.
Posit: Homosexuals claim that adult homosexual behavior is not a mental illness.
Conclusion: The huge majority of adult participants in homosexual behavior do so out of a voluntary choice (not genetic disorder or mental illness).

Posit: Society has the right to limit, discourage or attempt to prevent through law any voluntary behavior that is without societal benefit and detrimental/threatening to that society’s continued existence (through causing unnecessary, premature death or excessive loss of productivity of that society’s citizens).
Posit: Homosexual behavior has no societal benefits and directly causes the unnecessary, premature death and/or excessive loss of productivity of citizens (practicing homosexual activities) through extraordinarily high rates of AIDS/HIV and other STD’s and is, consequently, potentially threatening to a society’s continued existence.
Conclusion: Society has the right to limit, discourage or attempt to prevent homosexual behavior through law.

Posit: Society has the right to limit, discourage or attempt to prevent through law any voluntary behavior, not providing essential societal benefit, that is detrimental/threatening to that society’s continued existence (through the direct prevention of the production of new citizens for society).
Posit: Exclusively homosexual behavior provides no essential societal benefit and directly prevents the production of new citizens for society (from those homosexuals) and is, consequently, potentially threatening to a society’s continued existence.
Conclusion: Society has the right to limit, discourage or attempt to prevent homosexual behavior through law.

Posit: Society has the right to encourage through exclusive incentives and protect through law any voluntary human behavior that ensures or enhances that society’s continued existence through the continued production of healthy, normal, productive citizens to replace those who die.
Posit: The only consistent, voluntary behavior (statically valid and historically proven) for continuously producing the highest volume of healthy, normal, productive citizens for the continuation of a society (excluding immigration) is a family of an adult male and adult female in the stable, durable and protected marriage relationship.
Conclusion: Society has the right to encourage through exclusive incentives and protect through law the marriage of an adult male and adult female (and to enhance the stability and durability of their marriage relationship).

Posit: Exclusive incentives and legal protections encourage marriage of heterosexuals that benefit society through production of new, healthy, normal, productive citizens.
Posit: Making homosexuals officially eligible for marriage provides no benefits to society and reduces the exclusivity of the incentives to heterosexuals which weakens its desirability to this group and thus the attendant benefits to society.
Conclusion: Society should limit the exclusive incentives and legal protections of marriage to a stable relationship of an adult male and adult female.

Posit: Any human activity not required for the immediate survival of the individual (breathing, eating, etc.) is either voluntary or the result of a mental illness. Posit: Homosexual activity is not required for the survival of the individual. Conclusion: Homosexual activity is either voluntary or the result of a mental illness.

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...