But where in Article I does it give Congress power to define marriage at the federal level?
Article I Section 8 Clause 18: "To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof."
I should think that it is blindingly obvious that if Congress chooses to offer certain benefits for being married, that it is free to set whatever definition of marriage it chooses to without depending upon the States to do so. Certainly, it has never been held that States are the only ones who can determine what a marriage is. Indeed, in
Meister v. Moore, 96 U.S. 76 (1877), relating to common-law marriage, the court held: "Marriage is everywhere regarded as a civil contract. Statutes in many of the states, it is true, regulate the mode of entering into the contract,
but they do not confer the right." (emphasis added) That case held that unless a state statute expressly denied the legality of common-law marriage, it was to be held as valid for the purposes of the law of that state. It also pointed out that some states had expressly forbidden common-law marriages whereas others had not, hence explicitly recognizing that there was no requirement for uniformity in the definition of marriage.
If some states had passed a law defining tomatoes as fruit for their own laws and others had passed a law defining tomatoes as vegetables for their own laws, do you think the Supreme Court
Nix v. Hedden should have ruled that whether or not a vegetable was a fruit or vegetable for the purposes of Federal law depended upon which state the tomatoes were being imported to? I think not. Where different States may have different definitions of certain terms of law, it both prudent and proper for the Federal government to come up with its own definition for its own use. That is just as true for civil contracts for the importation of tomatoes as it is for civil contracts establishing marriages.
If DOMA is overturned, it will be on grounds that make same-sex marriage legal in all fifty states. There is no states' right argument to be made here, only a personal right argument.