And so what? Evolution, god, both, neither also resulted in humans engaging in mouth-to-mouth kissing, caresssing with the hands, penis-to-mammary contact, penis-to-mouth contact, penis-to-anus contact, vulva-to-mouth contact, hand-to-penis contact, vulva-to-mouth contact … how are these pairs any less “complementary”? Sure, they don’t result in pregnancy, but of what relevance is that?
The ability to brush our teeth is what separates us from animals 
This question has absolutely zero meaning,
No, there is no such goal.
Evolution happens. Period. There is no goal, no purpose, no impetus.
If a behavior or a physical feature does not create a disadvantage to survival until spawning, then it continues to exist until the species dies out for some other reason.
Sometimes. Sometimes the species dies out. I think we can pretty much take it as read that people are reproducing at a sufficient rate to avoid that for the time being.
I’m treating this more as an exercise in logic than a biological argument. Obviously, there is a specific biological function associated with penises, vaginas, and heterosexual sex. That is a trivial point to make. And if you want to claim that sex that doesn’t make use of that function is unnatural. That is an argument that is logically sound and holds water. If that’s what you want to argue, go for it. You won’t get any disagreement from me.
However, the logical corollary to that point is that “unnatural”, as a descriptor, therefore applies to ANY form of sexual activity that does not have reproduction as its goal. Again - if you’re willing to accept that consequence as part of your stance, then go for it. Perhaps that is magellan’s point of view. He hasn’t yet clarified if it is or not; at least not that I’ve seen. However, it you’re going to claim, for instance, that heterosexual sex while using birth control is “natural” and homosexual sex is “unnatural”, then I will say that logically, that’s a very difficult point of view to justify.
Complementarity is really a side-issue because, as has been pointed out, “homosexual” means something different from “dislikes penis-in-vagina sex.” I don’t think that whether the attraction is natural is either an easy question to answer or politically or legally important.
I’m not against homosexuality in any way, and quite enjoy engaging in heterosexual anal and oral sex. But people arguing that anal sex is every bit as natural as vaginal sex are nuts. That the vagina self-lubricates while the anus doesn’t is compelling evidence. (Also, the “wrongness” of it is part of what makes it so hot, for both parties.)
Pulling your quotes out of the quote box to answer individually.
is nonreproductive vaginal sex as “unnatural” as homosexual sex?
No, because the vagina self-lubricates.
What about heterosexual anal sex?
Yes. Deliciously so.
But really? Is that what’s behind the whole “homosexual sex is unnatural” belief? Because they have to buy lube instead of making it themselves?
Yes, exactly.
what about heterosexual vaginal sex that requires the help of artificial lubrication?
That’s a symptom of dysfunction, either something physically wrong or something psychologically wrong with that sexual encounter.
What about heterosexual anal sex? Are those more unnatural that homosexual sex? Less unnatural? Why?
Neither more nor less. Both are the same amount of hotly unnatural.
If a man has a really sweaty anus that acts as natural lubricant, is it then natural for him to be penetrated by another penis?
Does that work? I’m unconvinced, but wouldn’t completely rule it out.
And what about lesbians, anyway?
Not unnatural in any way, short of assplay.
Essentially, it boils down to penetrative anal sex is less natural than oral or vaginal sex simply because the anus has no natural lubricant while both the vagina and mouth do.
People mean a lot of different things when they call something “natural,” but I believe you are the first person I’ve ever met who included “self-lubricating” as part of the definition.
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Survival does not require that every single member of a species pass on its genes to the next generation.
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What is more numerical? 100 or 25?
Why are you being deliberately obtuse?
Getting back to the heart of the OP:
I rather like the example given upthread. When confronted with it’s not natural, respond with neither is driving a car.
It’s not deliberate. I’m genuinely not understanding what connection “self lubricating” has to do with something being “more” natural. It’s certainly more convenient that we evolved that particular adaptation, but we might very well have evolved with dry vaginas that needed to be lubricated with saliva before coitus. Saying that its more natural to have evolved that way is, at best, incoherent.
I already said this. It has a definition (it expresses a concept), but it doesn’t have anything close to a logical or well-defined meaning when it’s actually applied to nature.
And they often say it’s unnatural. The statement has nothing to do with how ‘natural’ it is; it’s an expression of disapproval that they phrase in terms of the natural word to attach some false authority to the claim.
We did skip right over that point. Homosexuality isn’t a type of sex act, it’s a sexual orientation. For some reason these sorts of discussions always return to one particular sex act.
Or perhaps more to the point, it means something different from “likes penis-in-anus sex”.
I’m a bit puzzled by the fixation on anal sex in this thread. Even accepting for the sake of argument that penis-in-vagina sex is natural and penis-in-anus sex isn’t, the latter is neither universal among nor exclusive to homosexuals. For reasons that I hope are obvious, it’s unusual for two women to have penis-in-anus sex with each other. While two men possess all the necessary equipment, there are gay men who’ve never had anal sex. And, as the graph on page 9 of this CDC report (PDF) shows, plenty of people have had heterosexual anal sex – more than a third of American women ages 25-44, and nearly half of American men in the same age group. There are in fact far more American adults who’ve had heterosexual anal sex at least once in their lives than there are American adults who’ve ever had any type of sex with a same-sex partner. Yet I’ve never heard anyone suggest that heterosexuality is unnatural just because heterosexuals engage in anal sex with each other.
Like I was just saying, you’re right. But this same thing happens in every single discussion of this topic. It does tell you where people’s minds are on the issue.
It seems to be a fundamental issue for some.
Shouldn’t the big question be, why does it matter? Cell phones, computers, eyeglasses and countless other things are unnatural. Why does it matter if men having sex with each other is natural or not?
The hoo-hah, unlike the bum hole, secretes a natural lubricant.
To be fair, the ass often has a lubricant. Just not one most people care for.
Because a lot of of people use the “it’s unnatural” argument to mean that type of sex is wrong and that same-sex relationships are wrong or just lesser than heterosexual relationships. The concept itself is irrelevant.