Whether or not marriage is a "fundamental right" I simply fail to see how the government can offer such a legal arrangement to some people, and not to others simply based on the sex of the participants. I suppose one could argue that they could just decide not to offer it to anyone, but I don't think people would like that much.
But as long as they're going to offer such arrangements it seems to me that the 14th amendment which says that no state can "...deny to any person within its jurisdiction the equal protection of the laws." mandates that it be offered to everyone regardless of who they want to be in that arrangement with (again with the proviso that all parties be able to give consent, be of the legal age of majority, and having legal standing).
I still haven't heard anyone give a good, non-religious, reason why everyone shouldn't have the same right to enter into the legal arrangement that, for now at least, the government calls marriage. The closest anyone has come is the procreation argument, but I don't think that really holds up very well.
@Philcore, I think we (mostly) agree on the fundamental issue, but since I'm an argumentative bastard I have to point out that I just don't personally think that the word marriage keeping this particular meaning is all that important. To me, at it's core, it's simply an arrangement between individuals whereby they agree to publicly state that they are in a relationship and would like to make that relationship binding to some degree, I just don't think the sex, race or anything else of the two people involved is really all that important or fundamental to what it is (at least by our modern conception of marriage).
But, I understand that the word is important (for some reason) to other people, which is why I think the government should just stop using it, and let churches, communities or whatever use that word however they like, as long as it has no legal ramifications.
Leaving out the legal arrangement a Church decided they wanted to marry same-sex couples who are we to say that that couple is not married within their Church's tradition? By the same token, if a Church doesn't want to marry people of the same sex, or people who've been divorced, or people who have cohabited before marriage, or people who don't want to go through the churches sanctioned pre-marital class or whatever, that's fine too (and these are all true of different Churches). But none of that should have any bearing on the civil legal matters.