Article 4, Section 1 of the US Constitution reads: "Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings 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."
The 1996 Defense of Marriage Act allows states to deny recognition to same-sex marriages legally performed in other states, which leads to problems in regards to inheritance, insurance coverage, and the legal status of married partners as next-of-kin. This law places unconstitutional restrictions on mutual state recognition and should be repealed. While a repeal is pending, the President should issue an executive order demanding that states comply with the Constitution.