Ditto. But, I think it is really a states rights matter.
Notning in the constitution says that a State can't require people to obtain marriage licenses. There are also state and federal anti-discrimination laws to contend with, which are Constitutional. As much as I hate gay marriage, I'd say that it will be difficult if not impossible for States to outlaw it.
If someone can explain to me how marriage is not a state's rights matter, I'd love to hear it.