As Rick Santorum takes the floor of the Senate to remind us that a constitutional amendment banning gay marriage is really about showing contempt for the sodomites, a who’s-who of conservative scholars (of whom, one can’t help but note, only a few seem to be social scientists) try a more respectable tack via the Princeton Principles, a survey of what the signers regard as the most important lessons about marriage to be gleaned from the social sciences and political philosophy. They conclude with five public policy recommendations—the first of which (naturally) is that any attempt to expand marriage beyond the one-man-one-woman formula (or even to create marriage-like alternatives for same-sex couples) must be resisted.

The form of the essay is, at this point, depressingly familiar. Pages and pages of mostly unobjectionable, uncontroversial stuff about how marriage is good for kids (along with some much vaguer and more dubious stuff about the purported inextricability of civil and religious marriage), followed by a weird series of logical leaps to the conclusion that gay marriage will spell doom for the institution.

That argument comes in four parts. They begin with some concerend chin-stroking over the fate of children raised by gay parents. There’s a lot of FUD about the need for more studies with larger samples and so on, yet somehow, somehow, the authors manage to get through an entire paragraph on the topic without mentioning that what social science evidence we do have points uniformly and strongly to the conclusion that kids raised by gay parents don’t tend to fare worse than their straight-reared counterparts, and that all the major medical and child welfare organizations have endorsed childrearing by gay couples. Since they won’t talk about the evidence we do have, they’re forced to speculate about the complimentary talents men and women bring to the parenting task—yet (as I’ve noted before) remain strangely mute about the beloved conservative point that, despite this supposedly vital complementarity effect, kids raised by remarried couples (controlling for income) don’t seem to do appreciably better than those raised by single parents. All of that notwithstanding, it’s not entirely clear why any of this is relevant: Whether kids raised in gay households are worse off than others is presumably a question that should affect how we prioritize prospective adoptive couples, or make custody decisions. It’s doesn’t tell us much about whether, given that some kids are going to be raised in such households, it would be better if their parents were able to marry.

We move on to Maggie Gallagher’s favorite point, that “Same-sex marriage would further undercut the idea that procreation is intrinsically connected to marriage…further weakening the societal norm that men should take responsibility for the children they beget.” That one doesn’t make any more sense to me now than it did last week, where I looked at some trends that at least suggest the causation doesn’t work that way. It is good if people who are going to procreate get married. It is fine if people get married who aren’t going to procrate. If there’s some tension between those two ideas, it’s too subtle for me.

The next might be my favorite: “same-sex marriage would likely corrode marital norms of sexual fidelity, since gay marriage advocates and gay couples tend to downplay the importance of sexual fidelity in their definition of marriage.” (Actually, as seems to be standard operating procedure in these discussions, “gay couples” seems to mean “gay men.”) Where the imperfect but substantial literature on gay childrearing is, apparently, insufficient to use as the basis of any conclusions—indeed, too thin to even be worth mentioning the results of—the authors are apparently prepared to make this assertion on the basis of one survey of the first couples to take advantage of civil unions in Vermont. And despite having offered a protracted argument for why the institution of marriage is so necessary as mechanism for cultivating norms of fidelity, the authors evince not even a sliver of curiosity about whether, if those norms seem weaker in groups that have heretofore been wholly excluded from that institution, those two facts might not be entirely unrelated.

Finally, the authors’ “concerns are only reinforced” by the growing acceptance of same-sex marriage abroad. Not because it has resulted in anything bad (you can be sure they’d tell us if it had), but because it “has taken hold in societies or regions with low rates of marriage and/or fertility.” This isn’t even a post hoc ergo propter hoc argument: It’s pre hoc ergo propter hoc. The idea, insofar as I can make it out, seems to be that if countries where marriage is viewed as less important have been among the first to let gay people in, then any country that lets gay people into marriage will come to view it as less important. Why we might expect this to be the case is, alas, not explained. I notice that both this argument and the “procreative link” one appear to rely on the presumption that “If A, then B” entails “If B, then A.” I think we may have discovered the real fountainhead of opposition to gay marriage: It’s not homophobia, it’s the inability to distinguish between a conditional and a biconditional. Which is a little odd, really: You’d think they’d like a logical operator that only swings one way.