In America it would fall under "pursuit of happiness" I suppose. I think the state has an interest in that partnered people are more likely to help each other when down and not need government assistance.That's probably true. But to play the devil's advocate, why should everyone have a right to choose their spouse? Why should anyone? Why is it a "right" at all?
As a thought experiment, what about marriages between close relatives or siblings? Australian law likewise "bans" marriage between people who have direct ancestry and siblings, biological or adoptive. Do you think that's a fair limitation on the definition of marriage? And if so, how would you justify this limitation? Is it because incest is "icky"? Is it because children conceived have higher probability of genetic diseases? Is it because of the disparate power relationship between relatives? Is it because doesn't affect as many people as ban on gay marriage? I think that if you break down each of the objections that you migth have (or that you might not, since I have no idea if you would approve of incestuous marriages), it becomes very similar to the objections that mojorising has against gay marriage.
I think the problems of genetic liability and power imbalance are real.
However, modern science can mitigate most of the genetic issues (screening, etc) and power imbalance can be mitigated with other social structures (and exists in many "approved" marriages already).
In general I don't actually have a problem with incestuous marriages. I would find it to extremely icky to be in one, but have no good reason in modern society to ban them.
Personally, I think all marriage should be "banned" and treated solely as a contractual matter. But since I'm in a hopeless majority with my opinion and marriage, I'm not losing sleep over some people being slightly less advantaged in this regard.
I agree with this, too. A "domestic partnership" aka marriage is a legal structure that benefits society between many types of couples including siblings, non-romantic partners (elderly widows, e.g.) and elderly parent with caretaker child. It's a system that protects assets, caregiving and decision making. I am for it and have no problem calling it "marriage" and also have no problem with a second name "domestic partnership" as a nickname of sorts for marriages where the partners wish for some reason to make it clear that they are not intimate. But you don't need a separate structure. Just "get married" and then use the term "domestic partnership" whenever you want to.