Ezra Klein complains that Scalia commented from the bench that "there's considerable disagreement among sociologists as to what the consequences of raising a child in a single-sex family, whether that is harmful to the child or not." According to Klein, Scalia should've been disabused by this notion because the American Sociological Association filed an amicus brief taking a definitive position on the question.
But Scalia's comment surely was prompted by page 12 and footnote 5 of the National Association of Evangelicals amicus brief.
Now, without reading the underlying cited studies, on a question of sociology, I'd be personally inclined to favor the ASA over the NAE, though both are far more politicized bodies than either would care to admit. On the other hand, sociology isn't an especially hard science. That said, if one side says "There's no disagreement" and the other side says "There's disagreement," complete with cites to disagreeing studies, it would seem to me that the side claiming disagreement is technically correct, even if one side of the disagreement has a stronger argument.
Frankly, given the very short history of gay adoption, it seems very improbable that anyone has a definitive answer on the effects of gay adoption. My null hypothesis is that two well-adjusted gay parents are going to do just as good a job as two well-adjusted heterosexual adoptive parents and better than a single-parent household or an orphanage. The gay parents I know are well above average. But there simply isn't any legitimate long-term empirical data on the question one way or the other—especially given that the gay couples who have adopted in the last twenty years are likely to have considerably different demographics than the straight couples who have adopted, making ceteris paribus judgments difficult.
Klein complains that "Scalia offered no details or evidence of this considerable disagreement among sociologists" which is a ludicrous standard for a comment from the bench during oral argument. Klein's attack on Scalia for disagreeing with Klein's opinion and the ASA is entirely unfair.
The existence of disagreement is relevant if the question is whether anti-gay-marriage laws have a "rational basis." One can legitimately question whether that should be the legal standard for evaluating laws discriminating against gays; there's a good argument for what is known as heightened scrutiny. One can also dispute (as some libertarian lawyers do) whether courts should be so deferential to legislatures in conducting rational-basis review—though Klein and liberals would surely throw a fit if the Supreme Court favored economic thought in striking down minimum-wage and rent-control laws as lacking a rational basis the way Klein asks the Supreme Court to give the American Sociological Association veto power over a legislative judgment. Simply put, under normal "rational-basis review," if conducted as it would be under any other constitutional challenge to any other kind of statute, anti-gay-marriage laws are constitutional. Lots of dumb laws I don't like are constitutional when subjected to existing rational-basis review doctrine.
Disclaimer: as you know, I support legislative initiatives for marriage equality, and have otherwise spoken out in favor of gay rights.
Commenters note that it's possible for one side of a disagreement to be inherently bogus. This is correct. But as much as I want it to be true, it's simply not the case that "Gay adoption is harmless" is at the level of scientific truth of "The earth revolves around the sun" or evolution. Neither side has definitively proven its case. Over time that will mean we can be more and more confident that the null hypothesis is correct. If I can place a wager where the social science will be in fifty years, I'd certainly lay odds that the null hypothesis is correct. But it's simply not the case that, because of a handful of studies subjectively measuring a few dozen children without adequate controls, we know for a fact that there's no long-term effect.
I'm sympathetic to an argument for requiring legislatures to be forbidden from creating restrictions on freedom of choice in the absence of scientific truth, but the simple reality is that, under current law, states have that right to ban sports betting and poker, to ban prostitution and Four Loko, to require a $9/hour minimum wage, and to restrict gay marriage--assuming that current Supreme Court precedent that laws discriminating against homosexuals need only meet rational basis review hold. We presume as a matter of legal review that the legislature knows what it is doing. I certainly don't agree with the proposition that legislative bodies always know what they're doing; I don't like these laws; other people like some of these laws and not others; but, in the absence of reversing existing precedent, all of these laws are currently within the range of constitutional state regulatory power as a positive matter.
I'm comfortable with the legal argument that the premise and existing precedent is mistaken, and that courts as a normative matter should, under the Equal Protection Clause, give more scrutiny to discrimination against homosexuals. I'm open to the argument that courts should have the power to use rational basis review in a more searching fashion, though it would be a big shift in the separation of powers and would politicize our courts far more than they are politicized now. And I disagree with Scalia. But he's not being intellectually dishonest to come to the opposite conclusion within the existing legal framework and precedent. The case for stare decisis in this area is far stronger than the case for stare decisis in any number of areas where liberals insist conservative justices cannot act because of earlier decisions.