Nelson Lund, writing in the Wall Street Journal:
A significant number of organizations representing social and behavioral scientists have filed briefs promising the court that there is nothing to worry about. These assurances have no scientific foundation. Same-sex marriage is brand new, and child rearing by same-sex couples remains rare. Even if both phenomena were far more common, large amounts of data collected over decades would be required before any responsible researcher could make meaningful scientific estimates of the long-term effects of redefining marriage.
The conclusions in the research literature typically amount at best to claims that a particular study found "no evidence" of bad effects from child rearing by same-sex couples. One could just as easily say that there is no reliable evidence that such child-rearing practices are beneficial or harmless. And that is the conclusion that should be relevant to the court.
There’s no way of knowing what combination of these singular features of marriage confers which of its demonstrated advantages, culturally and psychologically. We do know, however, that if the state suddenly creates the institution of gay marriage by fiat, the result will lack most of the features that make marriage unique—and uniquely beneficial. It will not be the same institution that has won the unanimous endorsement of social scientists. It will be a novel and revolutionary institution owing its existence to the devaluation of an old and settled one. Should we assume that the former will confer the same social and personal benefits as the latter, the two being different in such fundamental ways? The only honest answer—the only intellectually respectable answer—is, Who knows?