Here’s the basic thing you need to know (from NRO’s The Corner):
In [SCOTUS’s] ruling on the federal Defense of Marriage Act (DOMA), the Court struck down Section 3, declaring that the federal government cannot define marriage for its own federal policies and federal laws but must accept whatever the states decide about marriage. The Court’s ruling, however, does not affect Section 2, which provides that no state is required to give effect to another state’s recognition of same-sex marriages.
So they basically said state law determines what marriage is in that particular state. Marriages that occur in NY or Mass. are not recognized in Ohio, for example, because Ohio doesn’t recognize same-sex marriage.