Even though SCOTUS said this, I'm going to disagree. Article 1, Section 8 specifically authorized Congress to set standards for 'weights and measures'. 'Measures' has always been recognized to include legal definitions to ensure consistent application. As long as marriage is a legally recognized contract, defining it falls under that.
Still think DOMA needs to be raised to an amendment, though, otherwise the Enemy will succeed, with pervs already on the Court and ready to drop the gavel on the 97% heterosexual majority.