It is DOMA Section II that now remains that treats legal marriages between same sex couples differently than all other legal marriages. First cousins and 16 year olds married in one state are still married in the other stares that do not allow these marriages.
Under Article IV, Section 1 of the U.S. Constitution, states are supposed to give “full faith and credit” to the laws and court rulings of other states, subject to some regulation by Congress[...]
In Section 2 of DOMA, Congress carved out an exception to “full faith and credit,” excusing states from their obligation to honor the laws of other states “respecting a relationship between persons of the same sex that is treated as a marriage under the laws of such other State.” But while some lawsuits have challenged Section 2, plaintiffs have had trouble getting past the threshold question of standing. To show sufficient injury, presumably a same-sex couple will have to marry lawfully in one state and then relocate and establish residency in a state that refuses to recognize their relationship and, as a result, denies them benefits and access to state programs. Only then might plaintiffs have sufficient injury to even be able to challenge Section 2.
SCOTUS for law students: The Defense of Marriage Act and the Constitution (sponsored by Bloomberg Law) : SCOTUSblog
Thanks to DOMA III being overturned, more and more couples are going to be able to demonstrate "sufficient injury"...
It has everything to do with DOMA.