The Defense of Marriage Act (1996) means that the federal government does not view gay marriage as legitimate, and that states are not required to accept gay marriages in other states as legal.
"Four states (Massachusetts, Connecticut, Iowa, and Vermont) currently allow same-sex marriage (with Maine and New Hampshire having passed not-yet-implemented legislation to join that list), five states recognize some alternative form of same-sex union, twelve states ban any recognition of any form of same-sex unions including civil union, twenty-eight states have adopted amendments to their state constitution prohibiting same-sex marriage, and another twenty states have enacted state-level statutes to the same effect." (Wikipedia)
"Four states (Massachusetts, Connecticut, Iowa, and Vermont) currently allow same-sex marriage (with Maine and New Hampshire having passed not-yet-implemented legislation to join that list), five states recognize some alternative form of same-sex union, twelve states ban any recognition of any form of same-sex unions including civil union, twenty-eight states have adopted amendments to their state constitution prohibiting same-sex marriage, and another twenty states have enacted state-level statutes to the same effect." (Wikipedia)