Post below revised a bit from first version for clarity.
I am, alas, inclined to agree with Healy: The post series began with some reasonable-enough throat-clearing about the historical link between marriage and procreation, but rapidly degenerated (beginning with the suggestion that "death by sexual disorganization" caused the fall of the Roman Empire), by the time she reached her penultimate and final posts, into a truly phenomenal collection of non-sequiturs. That this seems to have been the opinion of most of the Volokh commenterers is, rather self-flatteringly, taken as evidence that "the wall is still up pretty high" against the "air and light" she's offering. (Continued after the jump…)
What's most interesting is that, while Gallagher purports to be making the case against same-sex marriage, there's very little there there: We go from throat-clearing about the general importance of marriage to postscript, with a bare handful of sentences devoted to what one would expect to be the crux of the argument. The throat-clearing bits mostly have to do with establishing that a major historical function of marriage has been the regulation of procreation and child-rearing, and that it's ceteris paribus better for children to be raised by a married couple than by single or even cohabiting parents. She further argues that it's better still if the married couple are the biological parents of the child they raise—something that's plausible enough on evolutionary psychology grounds as a statistical generalization.
Let's grant all those premises as generalizations; they still don't get Gallagher remotely near where she's trying to go. To the extent it's possible to extract a straigthforward argument from this series of posts, it's crucial to it that the regulation of procreation and child-rearing be, not merely a major public purpose of marriage, but the purpose.
Now, that's just demonstrably false. Gallagher cherry-picks some case law asserting the importance attached to this function of marriage, but omits, for instance, Loving v. Virginia's emphasis on the freedom to marry as "as one of the vital personal rights essential to the orderly pursuit of happiness by free men." In her final post, Gallagher imagines the (apparently ridiculous) situation in which business partners marry as an economic arrangement, without any particular love or intimacy between them. But, of course, nothing in current law precludes (different-gendered) partners from doing just than. And, as I noted last month, there have been times and places where marriage was seen, above all, as about cementing a business partnership and regulating household division of labor—or establishing trade between nomadic groups, or uniting disparate landholdings, or regulating inheritance, or cementing political alliances. As Amptoons observed a few years back, even contemporary pro-marriage conservatives had (until SSM became such a bugbear), a list of six "dimensions" of marriage, of which child-rearing was but one.
Gallagher chastizes Andrew Sullivan for raising the inconvenient point that, in fact, we don't narrowly-tailor the legal institution of marrriage to make it centrally or exclusively about procreation. Certainly, we could: Infertility has, in the past, been seen as natural grounds for dissolution of a marital union. If we wanted to really hammer home that all-important link between marriage and child-rearing, we could make it available only to those with a declared intent to conceive a child or adopt. Of course, with a rapidly growing number of gays raising children and half of the rest indicating a desire to someday, this would fail to exclude gay couples systematically unless we allowed marriage only between pairs who intended to biologically conceive a child genetically related to both parents—and even then the exclusion would probably not last much past a few advances in reproductive technology.
Gallagher seems to think that arguments of this sort are attempts to prove that marriage has no (public or legal) functional connection to procreation. But that's, one, ludicrous and, two, far more than someone like Sullivan needs to show to explode Gallagher's argument. All that's required is what's frankly boringly obvious in both history and law: Marriage serves a wide variety of private, public, and legal functions; child-rearing and procration are among them but far from exhaustive; and the institution as already structured recognizes this multiplicity of function. So why exclude gay couples, who certainly might raise children, and in the case of lesbians are capable of bearing them as well, but admit all heterosexual couples, whether they intend to or are even capable of procreating or raising children? According to Gallagher: "Because the way it works in reality is, the more people attracted to the opposite sex who enter such unions, the better off children will be."
That's it. Recognize that this is the crucial turning point in the argument—this is where her case stands or falls. Gallagher could defend a version of marriage that's more narrowly and strictly linked to child-rearing and procreation, though that wouldn't really allow her to draw the clean gay/straight boundary she wants. Instead, she moves to a distinction based on affective orientation rather than either procreation or child-rearing, asserting that this, too, is somehow "better for children"—presumably even after taking into account the children whose gay natural or adoptive parents can't marry their partners—but without any hint of explanation.
Now, we can make some sense of this if we see it through a sort of Straussian lens. As I suggested previously, the idealization of marriage, and much of its appeal, turns crucially on its serving those other functions: promising fulfillment and intimacy for the married couple, rather than just a stable childrearing environment. It's not that SSM threatens to disconnect marriage from procreation; the argument for gay marriage is appealing precisely because people already understand that marriage has meanings and functions beyond procreation.
Maybe what Gallagher says here is an indication that she recognizes this. She's effectively saying: Look, we can't have marriage just be about procration (and design law accordingly); its appeal involves all these other things that induce people to do it. Indeed, the people for whom (and for whose children) its most important are precisely those who demonstrably don't feel the need to get married just to provide a stable environment for their kids. Those people need, if you will, the marriage fairy tale. This entails a recognition that, at least for many people, marriage is not conceived as (exclusively) a procreative institution, even if its serving that function is what explains its universality. But that's the end of the game for Gallagher, or should be. You might think that providing a stable structure for linking kids and biological parents is the most important function marriage serves, but Gallagher's own argument against Sullivan (not to mention, as she might put it "all of human history") make it crystal clear that it can serve this function without the parties to it generally conceiving that as the sole or even primary function. Indeed, as her argument recognizes, it might very well serve that function better if people don't see that as the exclusive or even primary function.
In short, Gallagher wants it both ways. At some level, she understands that since the 18th century shift away from marital pairings determined by extended families, by a strong-handed paterfamilias, or by the broader community, the already existing conception of marriage as (inter alia) a vehicle for romantic fulfillment is necessary to the preservation of the institution as a sufficiently broad one. But she wants to avoid the logical consequences of people's thinking about it that way. I don't think she can get both.
En passant: We also get, in Gallagher's final post (the low point of the series, with the highest non-sequitur/text ratio) the assertion that SSM advocates want to "the use government power to impose a new morality on a reluctant people." The idea here is that there are too many cultural accretions of marriage to extend the legal institution to same-sex couples without appearing to give some kind of endorsement—conferring the cultural good vibes no less than the legal privileges. Gallagher doesn't seem able to see that the situation's perfectly symmetrical in this regard: So long as marriage (which, as she observes, was not invented by government) is bound up with the legal and political institution, the law will "impose" in this fashion. State involvement in marriage has, at various points in history, entailed little more than the formal recognition of an antecedent, primarily religious ritual. Codifying one particular form of marriage in law locks out the kind of natural evolution we otherwise have seen and doubtless would see in the institution. If we did not see the state as defining "marriage" in terms of its cultural connotations as well as its formal legal benefits, we'd already have widespread "gay marriage."
As we come to Gallagher's final post, we find little more than a rehashing of what we've seen to date: Having married parents is a great good to children (fine), and admitting couples who can't (yet) conceive any child they might raise between themselves somehow degrades the procreative and child-rearing functions of marriage. But the crucial mechanism is, as per usual, never really specified: We're not told why allowing that marriage may serve non-procreative functions will undermine the institution. All we get are some highly disanalogous analogies. Since this—not whether marriage is generally good, not whether it's typically better for kids to grow up in the context of marriage than with a single parent—is the key point of disagreement, we're going to need something a lot better here to make the argument work.