To his credit, Douthat concedes the weakness of many of the common arguments put by his side. First, it is incorrect, he admits, that lifelong heterosexual monogamy is universal or "natural" (I take it that the relevant sense of "natural" is something like "deeply-embedded in human nature" rather than something like "consistent with physical laws"). Leave aside what would follow even if it were universal or natural - Douthat is quite correct that the premise simply can't be sustained. If anything, the default family arrangement for our species is polygamous and the default mode of child-rearing communal.

That doesn't entail that we should adopt arrangements that involve polgamy and communal child-rearing, but it certainly exposes the danger of the common arguments that Douthat rightly rejects. If such arguments from "is" to "ought", relying on what has been anthropologically universal or what is "natural" to us, were valid, then defenders of traditional same-sex marriage might be shocked at the implications. In any event, there is nothing universal or "natural" (in the requisite sense) about traditional or Christian marriage.

Showing a bit of wisdom that his political allies might learn, Douthat points out the following:

So what are gay marriage's opponents really defending, if not some universal, biologically inevitable institution? It's a particular vision of marriage, rooted in a particular tradition, that establishes a particular sexual ideal.

He elaborates this "sexual ideal" as follows:

[It] holds up the commitment to lifelong fidelity and support by two sexually different human beings - a commitment that involves the mutual surrender, arguably, of their reproductive self-interest - as a uniquely admirable kind of relationship. It holds up the domestic life that can be created only by such unions, in which children grow up in intimate contact with both of their biological parents, as a uniquely admirable approach to child-rearing. And recognizing the difficulty of achieving these goals, it surrounds wedlock with a distinctive set of rituals, sanctions and taboos.

The point of this ideal is not that other relationships have no value, or that only nuclear families can rear children successfully. Rather, it's that lifelong heterosexual monogamy at its best can offer something distinctive and remarkable - a microcosm of civilization, and an organic connection between human generations - that makes it worthy of distinctive recognition and support.

Again, this is not how many cultures approach marriage. It's a particularly Western understanding, derived from Jewish and Christian beliefs about the order of creation, and supplemented by later ideas about romantic love, the rights of children, and the equality of the sexes.

This is important, because Douthat is going to go on and argue, in effect, that the power of the state should be used to shore up this particular sexual ideal. The state should give special recognition to relationships that meet this ideal, possibly with tangible legal rights attached. It should do that by recognising as marriages only those arrangements that meet the ideal - or at least don't blatantly depart from it, as with same-sex marriages. But one immediate problem with this is that the sexual ideal which Douthat advocates no longer has unchallenged acceptance, even within the culture of the West. As he acknowledges, it has largely been superseded, even in the popular understanding of marriage:

Lately, it has come to co-exist with a less idealistic, more accommodating approach, defined by no-fault divorce, frequent out-of-wedlock births, and serial monogamy.

In this landscape, gay-marriage critics who fret about a slippery slope to polygamy miss the point. Americans already have a kind of postmodern polygamy available to them. It's just spread over the course of a lifetime, rather than concentrated in a "Big Love"-style menage.

This is exactly right. While marriage means different things to different people, and there are many ideals of sexual love, the more pragmatic, accommodating, "postmodern" approach to marriage that he describes does co-exist with the old Christian/traditional concept. Indeed, the "postmodern" approach has largely displaced the "Christian" approach in the popular imagination. It seems to me, what's more, that this is a good thing, a social advance. Why on earth should people be trapped in loveless marriages that no longer suit them? Reason has gradually been prevailing in this area.

And of course there might be still other sexual ideals that can find expression in personal styles of marriage. Two people may see their marriage as essentially about companionship. Another two might see themselves allied in a great life-adventure that they face together, which will involve them both gaining diverse sexual experience with other people. There are many possible ideals on offer.

In any event, marriage no longer acts as a means to regulate who can have sex with whom - while most people still frown upon adultery and polyamory, and some still frown on "fornication", all of these practices are perfectly legal in most Western jurisdictions. Whatever marriage now symbolises for this person or that person, it does not have any legal force that commits participants to monogamy, to exclusive heterosexuality, to lifelong union, or to procreation. While many people still want to get married, what it symbolises to them is now much more diverse and often rather inchoate. It is, as Douthat acknowledges, usually something to do with a celebration of romantic love, but it now has nothing directly to do with procreation or with state regulation of who is legally permitted to have sex with whom.

So Douthat is left with the bare argument that ...

lifelong heterosexual monogamy at its best can offer something distinctive and remarkable - a microcosm of civilization, and an organic connection between human generations - that makes it worthy of distinctive recognition and support.

... and that

if we ... accept this shift, we're giving up on one of the great ideas of Western civilization: the celebration of lifelong heterosexual monogamy as a unique and indispensable estate. That ideal is still worth honoring, and still worth striving to preserve. And preserving it ultimately requires some public acknowledgment that heterosexual unions and gay relationships are different: similar in emotional commitment, but distinct both in their challenges and their potential fruit.

But hang on. Douthat is entitled to honour his particular sexual ideal as much as he likes. Let him have a lifelong monogamous heterosexual marriage involving children. Fine. May his children grow up healthy and happy. Doubtless there are many other individuals who honour the same ideal, expressing it in their life plans and projects. Well, let them.

But nothing prevents that. On the other hand, the state now permits "fornication" and "adultery", and it recognises marriages that are deliberately childless, marriages that are open or involve other polyamorous arrangements (I could tell you plenty of stories just from the science fiction community in, say, the US), and doubtless all sorts of other things that I can't even begin to imagine. It also permits procreation outside of marriage, and has abolished old notions of "illegitimacy".

Marriage takes many forms, in modern societies, and is used in many different ways by many people with many diverse ideals. I doubt that there is much sense continuing to have a state-recognised status called "marriage" anymore, but insofar as we go on doing this the status has become extremely malleable. And again, this is a good thing. We are not all cut from the same template; we are all different, as individuals, and we should, as far as possible, be free to live in accordance with our varied conceptions of the good.

If same-sex marriages obtain recognition from the state, that won't prevent anyone from living in accordance with the ideal that Douthat espouses. But it's not good enough for Douthat to say that this is an attractive ideal that "we" should honour.

Whether "we", as individuals, want to honour it is up to "us" as individuals. If somebody wants to live in accordance with a sexual ideal of lifelong, monogamous, heterosexual fecundity ... she should be free to do so. But she has no claim on the rest of us that her sexual ideal gets some special advantage from the state in competition with the many other the alternatives that are on offer. Yes, we as individuals can choose the ideal that Douthat loves so much. Douthat himself can, well, do whatever he likes, provided he doesn't harm others.

But as for whether the state should give such a sexual ideal some special honour ... why? In a pluralist society it is not the role of the state to give special honour to one or the other of the many different ideals (sexual or otherwise) that are legitimately available to people, and which, in effect, compete for our adherence. The state should allow people as far as possible to live in accordance with their diverse views of the good; it should not honour one particular group's view of the good and, by implication, stigmatise another's.

Of course, Douthat is welcome to argue that there is some good secular reason not to provide for same-sex marriage because, for example, it will cause suffering, or because it will lead to social breakdown and civil chaos. But he hasn't even attempted to put an argument of that kind. His argument is, instead, the illiberal one that the state should give its backing to his particular, entirely optional, sexual ideal. But why the hell should it?

That's why it's a pathetic attempt to oppose same-sex marriage. When an intelligent conservative who opposes same-sex marriage looks at the issue squarely, he has to concede that the usual arguments relied on by his allies are rubbish. What's more, he clearly has no viable argument based on secular concerns, such as concerns about harms to worldly interests. Surely he'd put an argument like that if he had one: he knows that recognising same-sex marriages will not lead to suffering or chaos.

Douthat is left with a bare plea that the state should honour his favourite sexual ideal over others that compete with it. Sorry, sir, but that's pathetic.