The word marriage is never mentioned in the Constitution, not one time. I put the entire Constitution in an Excel sheet and did a lookup for the word marriage, and it's not in there, which according to the 10th Amendment makes this a states issues. This is why you get a state issued marriage license.
The problem with jackleg "federalism" reduced to the pernicious doctrine of "states rights" is that it leaves a couple of things out. Here's the part of the 10th Amendment that Kittilsen notices:
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively ...
And here's the part he leaves out:
... or to the people.
He also leaves out the entire 9th Amendment:
The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.
Federalism is not a uni-directional doctrine with two parties. It is a multi-directional doctrine with three parties (United States, states, people). And when any one of those parties steps on the toes of one of the other parties' prerogatives, the offended party is fully justified in kicking the offending party right in the shins, and invoking the third party's assistance in doing so.
The right to marry is clearly an unenumerated right "retained by the people." Marriage -- including same-sex marriage -- has been around since long before state "licensing," long before the Constitution, long before the state as we know it.
In the United States circa 1790, if you wanted to get married, you might go about it in several ways ranging from formal (consulting your church to get its blessing and have your marriage entered in its records) to casual (moving in together, commingling your property, and "holding yourself out as married" to the community around you). Asking for the government's permission was not one of those ways, and suggesting that it should be would likely have involved you in tar, feathers and so forth.
It wasn't until the 1830s that the states began unconstitutionally seizing the power to "license" marriages -- initially to stop whites and blacks from marrying each other, but incidentally embedding the dominant religious doctrine of the time (heterosexual marriage only) in its licensing schemes as well. That nonsense has been going on for 180-odd years now. It was wrong when it started and it's never stopped being wrong. It's just that one bad bit of it has only just now finally worked its way through the courts.
The proper SCOTUS ruling would have been to void all state "marriage license" schemes, as they are clear violations of federalism insofar as they unconstitutionally seize on behalf of one party (the state) as a power something which belongs to another (the people) as a right.
Barring doing the right thing, it wasn't that unreasonable for SCOTUS to at least rein in the states' illegitimate seizure of power and tell them that they don't get to wield their ill-gotten power prejudicially against one particular segment of the people (same-sex couples).