The Wife pointed out that my Marriage Hypothetical post was confusing. A fair criticism -- I took a background that will look like a law school exam and used it to pose normative questions. Of course, I was trying to provoke thought about same-sex marriage, too.
First, you need to decide whether you think it's a good idea to let cousins marry. Cousins marrying gets close to the incest taboo. Most of us would recoil from the idea of letting brothers and sisters marry -- and it's instructive to give some thought as to why that should be the case. Most people would say, "because the children are likely to have birth defects," which is a simpler way of saying their genetic makeup will become weak because both parents are too similar genetically to have a good mix and create viable offspring. But, what if they don't have kids at all? If you take the idea of kids out of the equation, aren't you left with "well, yuck!" when confronting the idea of brothers and sisters marrying. And cousins are not brothers and sisters.
I think the reaction of "well, yuck" isn't a good enough reason to stop people from doing something they really consent to doing. Just because you and I find what other people do to be icky does not mean that they should not be free to do it.
Then you might also come to a religious objection. The Bible says cousins shouldn't marry, right? Actually, I'm not sure if it does or it doesn't; it's not my holy book to interpret. Abraham seems to get away with it in Genesis 12:13, 17:15-16; Moses was the result of an incestuous marriage in Exodus 6:20 and he turned out to be okay; and I really don't get 2 Samuel 13:1-22, in which a man unsuccessfully tries to seduce his half-sister, then rapes her, and then is cast as evil for refusing her demand that they get married after the rape.
But that's besides the point. The point is, we don't know if the people in the hypothetical follow the Bible at all. Even if your holy book says what they are doing is wrong, maybe their holy book says different, and shouldn't they be free to practice and follow the teachings of their religion? If their religion says it's okay for first cousins to marry, then who are we to say otherwise?
Then the other normative question you have to ask yourself is whether it's okay for one state to say that a couple is married and for a different state to say, "no, you're not." The couple here is married under the law of one state (Utopia) but another state (Columbia) won't recognize the marriage. What does that look like in reality? You can't stop them from living together or having sex or calling themselves "married." But you can refuse to extend benefits like health insurance. That's what a state's refusal to recognize marriage does in reality.
Those are the stakes in the same-sex marriage fights to come. Those are the social and legal questions we have yet to confront.