As Caleb Crain pointed out way back in 2003, Antonin Scalia’s dissent in Lawrence v. Texas, which struck down Texas’s anti-sodomy laws, argues that the majority’s reasoning leaves the federal government without any constitutional way of forbidding gay marriage.
Today’s opinion dismantles the structure of constitutional law that has permitted a distinction to be made between heterosexual and homosexual unions, insofar as formal recognition in marriage is concerned. If moral disapprobation of homosexual conduct is “no legitimate state interest” for purposes of proscribing that conduct; and if, as the Court coos (casting aside all pretense of neutrality), “when sexuality finds overt expression in intimate conduct with another person, the conduct can be but one element in a personal bond that is more enduring”; what justification could there possibly be for denying the benefits of marriage to homosexual couples exercising “the liberty protected by the Constitution”?
I think Scalia meant to weight the contrapositive a little more than the direct implication, but hey, the two are equivalent, and it looks like he was right.
Update: Just to give this a little more context: It might look from this excerpt like Scalia is saying “gay marriage is obviously terrible and this decision would make it unconstitutional for Congress to outlaw gay marriage, therefore the decision is wrong.” But in fact Scalia is responding here not to the Court’s main opinion in Lawrence, but to Sandra Day O’Connor’s concurrence, in which she goes out of her way to explain that her vote here should not be taken as precedent for establishing same-sex marriage as a constitutional right, no way, no how; while “moral disapproval” is not a legitimate state interest, she says rather vaguely that “other reasons exist to promote the institution of marriage beyond mere moral disapproval of an excluded group,” and that these reasons, whatever they are, mean that laws forbidding same-sex marriage are constitutional.
Scalia’s dissent says, and I paraphrase, “You’re kidding yourself, Day-O — if there’s no legitimate state interest in forbidding sodomy to same-sex couples while allowing it for opposite-sex couples, then there’s no legitimate state interest in forbidding marriage to same-sex couples while allowing it for opposite-sex couples, and you shouldn’t allow yourself to pretend otherwise.” And he was right!
Quomodocumque 
Looks like a clear case of appeal to consequences.
In the same vein: in his dissent to the DOMA ruling, Scalia wrote the text that could be used in an opinion legalizing gay marriage.
Trying to understand the post – Did Terence make his comment before you made the update? Interesting post. I’d like read to read why Scalia think that moral disapprobation of homosexual conduct WAS a legitimate state interest.
And you are correct, Scalia was right.
Yeah, my update was sort of in response to Terry’s comment.