Holding that the Constitution gives exclusive power to the States to regulate marriage is at once silly and horrible public policy. Gays moving to those certain states will have to endue their marriages being effectively terminated from now until rocks cool. The whole idea of gay marriages being recognized in some states, and not others, simply is not sustainable over the long term, and raises disturbing issues as to whether the right to travel is effectively being eviscerated, with the burden on gays in traveling to some states just shocking to the conscience.
Plus DOMA is about cutting off federal benefits. Does Congress not have the right to decide who gets federal benefits, and who does not? Of course it does, unless there are equal protection issues. So we get back to equal protection. There is no escape from that.
Anyway, the point is that if States have exclusive jurisdiction, then Congress in the future will have no power to pass legislation that gay marriages must be recognized everywhere. Well, maybe they could withhold funds from states that don't, but if states have exclusive jurisdiction, then that raises undue burden or coercion issues on the States, which is an issue as to which Kennedy is particularly sensitive, as he opined in the Obamacare case.
SCOTUS would be wise to just butt out, and let this matter be resolved at the ballot box. We know now what the result will be if that is the case. There is no turning back.
Obviously holding that the states have exclusive authority over marriage would be silly. The thing is, that's not at all necessary to strike down DOMA. A perfectly reasonable ruling could just hold that so long as the federal government uses state-issued marriage licenses as its criteria for determining which couples get federal benefits, they should have to treat identical marriage licenses identically. The Federal government doesn't have to give anybody marriage benefits, but given that they have decided to grant such benefits, and given that they rely on the states to actually marry people, they shouldn't be able to give marriage benefits to one couple that's married in Massachusetts while denying them to another Massachusetts couple whose marriage is just as equally valid under state law unless they have a
very good reason for doing so. DOMA can, should, and probably will be struck down
without destroying the American model of federalism as we know it as some are suggesting.