Read the introduction to this series, first.
1) The nature argument. Marriage is not just about more intimacy. It is about merging with one’s sexual other half or counterpart, a complementary sexual other. Erotic desire for what one is as a sexual being is sexual narcissism or sexual self-deception: an attempt at completing oneself sexually through merger with a sexual same. Most people intuit something developmentally deficient about being erotically attracted to the body parts and essential gender that one shares in common with another. See my online discussion in “Why the Disagreement…?” pp. 30-46 here; and my published entry on “Homosexuality” in New Dictionary of Christian Apologetics (Intervarsity Press), 327-32.
To be able to say what marriage is “about” one must be able to argue for a clear definition of marriage. For this to be a valid secular argument the definition of marriage must itself be secular. Here, Gagnon uses an argument of complementarity which he doesn’t acknowledge comes from his own reading of scripture, but which he makes clear in other works he has written on the topic. Furthermore, it is easily demonstrable that the meaning of marriage is not of necessity linked to a belief in complementarity. One need only look at the differing patterns and purposes of marriage in a variety of cultures and historical circumstances to see that the meaning is variable. In some cases it is primarily a contractual arrangement between two kin groups, in others it is a matter of providing security for the paternity of children produced from the union. To see clearly how this is true read: Janice Stockard, Marriage in Culture: Practice and Meaning Across Diverse Societies, Wadsworth Publishing 2001.
An argument from nature is uncompelling for the additional reason that Gagnon seems to use the term ‘nature’ to mean, “philosophical categories he sets up and determines the validity of.” But complementarity as a philosophical category is not a given. It has its genesis (pardon the pun) somewhere and it must be argued why it is relevant not assumed. To say “most people intuit X” is not an argument it is hyperbole. I do not see any reason to agree that most people intuit anything and even if there were some poll or statistic to use as a prop for this shabby tactic there would be no way to demonstrate that people’s intuition wasn’t cultural indoctrination. Furthermore, even if we could assume some kind of pure intuition free from cultural indoctrination Gagnon gives us no reason why we should trust such an intuition. It might be correct to say “most people intuit that spiders are dangerous,” which by and large is untrue, or “foreigners are unreliable” which is an unverifiable generalization. It is a big step from intuition to truth.
In the article he links to here, he explicitly rejects arguments from biological science which is what many would regard as the basis for determining ‘nature’. This is convenient for him, because biological science at each juncture contradicts his viewpoint. If complementarity were true one would expect biological science to show that behavioral patterns other than the type of modern monogamous heterosexual marriage Gagnon wants to be normative are unequipped for survival. However, the sheer diversity of marriage arrangements around the world, including polygamy and polygyny, shows this is not the case. Furthermore, biological science has demonstrated that the genders, which Gagnon relies upon as the basic units of his argument from complementarity are not discreet biological roles, but are on a sliding scale. One can be, biologically speaking, more or less male and more or less female. One can also be genetically female, but with male genitalia and vice versa. Finally, biologists have observed nearly every species of mammal engaging in homosexual behavior and there is strong evidence to suggest it is a congenital condition not born of environmental factors. Now Gagnon has argued forcefully elsewhere for why we should not expect a biological explanation for homosexuality to override the scriptural witness, but when making an argument on secular grounds his dismissal of scientific evidence is much less convincing.
Basically, Gagnon’s argument here amounts to: Marriage as he defines it (in total ignorance of the diverse ways it has been defined historically or in other cultural contexts), is natural, again as he defines it (while disallowing scientific evidence to influence the definition). Therefore, all other forms of marriage are unnatural (which is bad).
To say the least, I am unconvinced.