Ross Douthat does a very fair job  of summarizing the Regnerus study on outcomes of gay parenting, and how that study can plausibly be used as the basis for arguments either for or against gay marriage. And I am inclined to agree that the consequentialist case for gay marriage – that it will change gay culture for the better, that it will strengthen marriage as an institution, etc. – is inevitably weaker than the consequentialist case against – a case which says, basically, that since you don’t know what the outcome will be you should move very slowly and incrementally in implementing any change.
But to my mind, this just points to the limits of consequentialism. The precautionary principle, if taken really seriously, is an argument for never doing anything. Social science is never going to be able to tell us enough to confidently endorse changing social arrangements. When Zhou Enlai was asked what he thought was the significance of the French Revolution, he answered, “it’s too soon to say.” That’s surely the right answer to any question about what the impact of gay marriage might be – and will probably be the right answer forever, if only because of the extraordinary number of confounding variables.
My own instinct is that, yes, growing up with same-sex parents poses some interesting psychological challenges. If you are a boy raised by two women, for example, you may need to look elsewhere to find a male figure to identify with. But plenty of straight fathers do a lousy job of role-modeling. Moreover, there are innumerable other circumstances that pose unique psychological challenges, more obviously serious than being raised by a same-sex couple: being raised by an elderly father who dies during your adolescence (I have several adult friends who lost parents in adolescence; all of them were shaped profoundly and permanently by the loss); being raised by young parents whose marriage does not survive the inevitable strains (my parents married right out of college, and split when I was seven); or being raised by people who are not your biological parents at all (I am an adoptive father). We do not have a great national debate about whether to prevent older men from marrying younger women, or whether to prevent further encourage delaying marriage (rather than opposite – social conservatives fret that we are marrying too late on average), or whether to curtail adoption (though there are dissenters, the overwhelming consensus on left and right is in favor of adoption as a humane response to the great number of children in need of stable homes). What’s special about gay couples that requires them to clear the bar of ideality?
The law is not designed to make sure that every child is raised in an ideal home, nor even to make sure that our homes steadily approach some asymptotic ideal. The law is designed to protect children from situations of abuse and neglect, and otherwise to preserve the peace and to facilitate the social arrangements that the citizenry finds natural and sensible. It is the citizenry, acting through its representatives, that should decide what is natural and sensible, which is why I feel the legislatures of the several states  are the right venues for deliberating this question, but the point is: the deliberation isn’t really about social science theory, but about sociological reality.
The case for gay marriage – the Burkean case, you might say – is simply that what amount to common-law gay marriages already exist. Numerous gay couples settle down for long-term, even life-long relationships of mutual support. They jointly own property. They bear, adopt, and rear children. These are already existing realities, not hypotheticals. They are not the product of state diktats; they are the product of organic cultural change which, in turn, has shaped changes in the law. The question before the people is whether to recognize these realities, and, if so, as what. “As marriage” is one answer – the answer favored by those who want to secure those already-existing arrangements, for families already in them and for future generations who might want to form similar arrangements. And it’s the answer that seems to be getting intuitively more persuasive to more and more people as they look at these couples and at straight marriages and don’t see any fundamental differences that the law should be cognizant of.