Naturalistic fallacy

At Iain's* blog, he writes:

It is not tradition that defines marriages primary purpose but our biology.
Human beings like all of the great apes give birth to young that are incredibly dependant upon their mothers for a very long time. To successfully raise their young Human beings rely upon a pair bond between the mother and father of that child to share the care and nurture. It makes biological sense that a male should want an exclusive relationship with the female because then he can have some level of certainty that the offspring he helps nurture will be those to which he has contributed his DNA. As a human female is ready to produce more young before the first is mature it makes biological sense that subsequent offspring should be by the same father as this will extend the duration of the child rearing bargain between them.
It takes no stretch of the imagination to realise that formalising this arrangement with rituals and cultural practice and we call that formalisation marriage.
There fore if marriage is the formalisation of a biologically defined activity(reproduction) then the biological activity(reproduction) is the primary purpose of marriage.

Brigid Gread responds:

Actually, Iain, since you choose to base your argument on the premise that marriage is formalisation of the biological need to procreate, you certainly are advocating polygamy. According to psychologists, anthropologists and zoologists the world over, polygamy is nature's way of ensuring genetic survival; very few species are naturally monogamous because the male procreates and moves on in order to increase the likelihood that his progeny will survive. Dr Robert Winston's BBC series was very big on the idea that human monogamy is highly artificial and runs contrary to instinct. So, suggesting that marriage is primarily biological is rather paradoxical; sociologically it's little more than a social ritual, and on that basis the gender of its participants really shouldn't matter.

None of this is to suggest that I support polygamy, mind you. But if you want to play the 'biological necessity' card, then be careful where it might lead you.

Quite. Many animals (lions, for example) will practice infanticide in the interests of gene preservation.

Anyway, Iain's reasoning here is faulty on several other counts:

1. He gets the biology wrong when he asserts: "It makes biological sense that a male should want an exclusive relationship with the female because then he can have some level of certainty that the offspring he helps nurture will be those to which he has contributed his DNA." It's the gene that is "selfish," remember, not the organism. From that point of view, as Brigid correctly argues, polygamy is a far more efficient method of gene preservation than monogamy. (Besides, prior to the 1950s, no-one knew they were contributing DNA to anything!)

2. He is begging the question: insofar as in order to prove a link between marriage and biology, he presupposes it.

3. Given that the same-sex marriage debate is concerned with marriage laws (and whether and how they should be altered), the "primary," "secondary," "tertiary" and even "quaternary" purposes of those laws are whatever the drafters and interpreters of those laws happen to think they might be. It has nothing to do with biology (which is to say, the purposes of marriage laws, whatever they may be, are not biologically determined). And laws can be changed (just as they withe respect to the legality of homosexuality.

4. Even if we were to accept that the "primary purpose of marriage is reproduction," and I for one don't, wouldn't this be at least an argument in favour of allowing lesbians to marry--given the reproductive technology now available to them (e.g. sperm donation, IVF, etc.)?

(*In spite of the fact that this is the second of two consecutive posts taking issue with Iain's remarks at his blog, I am not waging a "bash Iain Hall" campaign!)