Here is what I see as wrong with both arguments: they conflate two distinct types of causation.
The historical cause of what might encourage or discourage the spread of a given conclusion among a society is the first type, the type with which the socio-mechanistic arguments are concerned. The logical cause, which the individual in his own mind uses to decide whether he will adopt or reject the conclusions of the society around him are a different type of cause. The socio-mechanic feels, once he has discovered a link between the conclusion and a possible benefit (such as an increase in population numbers, or a decrease in birth defects) that no further investigation of the reason is needed, even if the benefit is not one sought by the man doing the reasoning.
I remember the day and hour when I, a perfectly tolerant libertarian, rejected (with revulsion) the notion of gay marriage, and, in so doing, was logically required to reject toleration for homosexuality. It was March 05, 2002, at 10:00 in the evening. I was watching a television show where two lesbians were helping a bride get ready for her wedding. The bride spoke in the most glowing and romantic terms about the nature of true love: the two lesbians started making bedroom eyes at each other and smiling, for it was the intent of the writer to put across the idea that two lesbians having "sex" (i.e. masturbating with each other) was morally and logically the same as a bride and bridegroom having "sex" (i.e. consummating their wedding, and generating progeny and creating a family).
While I was (hitherto) willing to accept the libertarian argument that perverts should be left alone to practice their perversions, so long as they harm none but themselves, the liberal argument that true love is perversion and perversion is true love was so shocking to me that I was thunderstruck to the core of my being.
I felt toward homosexuality the emotion describe by G.K. Chesterton in his book, THE EVERLASTING MAN: "It is not true to human nature or to common sense. Let any lad who has had the look to grow up sane and simple in his day-dreams of love hear for the first time of the cult of Ganymede; he will not be merely shocked but sickened."
That was when I realized that the perverts did not want merely to be left alone: they wanted the same sanction, applause, and good wishes poured on their mockery of holy matrimony, that the courts and the church bestows on holy matrimony. Logically, a thing and its mockery cannot be granted both the same approval.
Such were my reactions, thoughts and conclusions. Here is the overwhelmingly obvious flaw in the socio-mechanical argument: at no point in time did my contemplations turn on the question of whether the world's population figures were "too high" or "too low" for any particular given purpose. Higher population may be good for some things (more workers) but bad for others (more mouths to feed).
Merely informing me that the world population is "too high" for some purposes, would have no particular affect on the chain of reasoning of mine, or of any sane man. Homosexual "matrimony" is a blatant contradiction in terms: the population figures in China do not make it somehow not a contradiction. Smothering babies at birth also would tend to lower the population, as would killing grandparents, arranging a famine, or starting an atomic war: the "taboo" against infanticide, patricide, famine and atomic war is hardly explained by such goofy explanation.
The same objection applies to the argument that ancient peoples noticed the medical problems of inbreeding, and deciding to adopt revulsion towards incest into human law and custom. A more obvious explanation, and one not so mechanistic, is that erotic love, by definition, is exclusive: in a wholesome heart, cleaving to one sexual partner excludes all others. The second wife is always jealous of the first wife, the husband always jealous of his wife's other lover.
But the love of brother to sister and mother, the love of father to daughter, is not exclusive: it is family love, and it has a different nature from erotic love. The two kinds of love are antithetical to each other: the admixture of erotic love into family love would ruin it. One need only try to imagine the difference between a mother loving two of her sons, or a brother loving two of his sisters, and a mother having sex with two of her sons, or a brother having sex with two of his sisters, to see the difference. It is blindingly obvious that the human heart (in sane and wholesome humans) cannot mix the two types of love, because the nature of the human heart makes this the case. It is not obvious at all, that ancient peoples did statistical studies on the genetics of brother-sister marriages, saw the social disutility and health problems of the practice, and determined by fiat to implant the custom of revulsion toward incest in our hearts.
I found on the web another example of the socio-mechanical argument: this author says that we men like beautiful women because our genes seek a healthy mate, and that this is "unfair." Like other socio-mechanical arguments, it explains without explaining. (see http://home.san.rr.com/denbeste/beautiful.html for his text)
For example, I am fascinated by dark-haired, short, dark-eyed women, particularly Japanese women. I suspect the root of my attraction has more to do with silken kimonos, the catlike tilt of eyes of elfin mystery, the gentle politeness of oriental females, and admiration for other aspects of the Japanese culture, than any evolutionary calculation in my genes that mating with a yellow woman would produce offspring more able to fight off heart disease than mating with a white woman.
The problem of all such arguments is that they reduce the contents of one's consciousness (something all men are aware of) to an epiphenomenon, a by-produce, of inhuman mechanical and biological forces, something no one is aware of. As such, the socio-mechanical argument is always mildly offensive.
And also silly. What I find particularly comical about this argument is that it attributes human notions of fairness and unfairness to inhuman mechanical processes.