Sorry to go waaaaaaay off topic (well not waaaaay), but a couple things strike me as relevant:
Marriage has traditionally been viewed by apostolic Christians as 1 man + 1 woman. A big part of this is having children and raising them in the faith and peopling the planet, but part of it (the most secular part, perhaps) relies on the simple fact that the family is a mini-government. How many times did we here our moms and dads pronounce, "This is NOT a democracy" after all? :-) The family is where the littlest citizens learn to be citizens, where they are introduced to their moral and civic duties, and where their needs are attended to - and for that reason, the state tends to recongize it as special. My dear orthodox Jewish friends assure me that marriage is a gift from God to help his people live in peace. There are certainly enough statements in Scriptures in support of marriage and the married.
Things other than marriage have been eyed with great suspicion in our culture, given its Judeo-Christian roots. Remember the famous Lee Marvin "palimony" suit in the 1970s? It was so fascinating because his live-in got something like alimony when the relationship ended. You see, it used to be that no one recognized a person as deserving anything at all after the break up of a non-marital relationship. Why? Well, because marriage is a contractual type relationship, special enough to be recognized by the state, and we understand that it is based on several things, not all of them snuggling (as married people, esp. those with kiddies know - that's just one aspect) and those who were "living together without benefit of marriage" folks were not recognized has having a legitimate contract between them because (the argument goes) when you strip away the trappings and responsibilities of marriage, all you've got left is the snuggling. And a contract based upon snuggling is akin to prostitution and we don't recognize immoral contracts. Indeed, if you look at some of the old common law marriage doctrines in that light, you can see how that the state went out of its way to find a marriage existed, even without benefit of an actual ceremony.
I don't want to tell others how to live their lives. But I do think that marriage is special. It isn't living together (it's much more than that). If people want to have special relationships for other purposes - like, as someone mentioned, for health care coverage - then that is something we can fix by not tying those benefits to marriage itself. No need to fumble with what marriage is, though, because that is far too divisive and wrong.
