First and foremost the Defense of Marriage Act (DOMA) violates the Full Faith and Credit Clause of the Constitution. If a driver’s license in one state is valid in another, than a marriage license or certificate denoting a civil union should as well. Article IV, section 1 of the Constitution, the Full Faith and Credit Clause states:
Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceeding of every other state; And the Congress may by general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof. No judicial precedent has been set on how this clause applies to same-sex marriage. So, opponents to same-sex marriage argue that Congress’ general authority to “prescribe…the effect” of public acts arguably gives it discretion to define the “effect” so that a particular public act is not due full faith and credit.
(Moreover, DOMA violates the equal protection and due prcocess clauses of the 14th amendment for the reasons stated throughout this blog.)
A History of the Legal Standing of the Definition of Marriage 