In the mean time, I'll just offer a question (or maybe 1½ questions) for everyone here:
Why should the institution of marriage exist at all? What is its purpose?
Well, this provoked some responses, including this comment by gvdub
Marriage is not, nor has it ever really been "an institution". Speaking of it as such is rather disingenuous. Marriage is, and pretty much has always been a contract. Historically, it was commonly referred to as the "contract of marriage".
Why should it exist? 1) As a framework to provide security for both partners and any offspring. Things like insurance benefits, estate matters (if one of a same-sex pair dies intestate, their partner is pretty much hosed, no matter how long they've been together or how much they shared in building whatever they have. Even with a will, but no legal marriage, if the family of the deceased is determined, they can pauper and humiliate the surviving partner, who has no real legal rights without that legal standing. ...
2) To provide a concrete reminder of the commitment that two people make to each other. It's easy, in the heat of an argument, to walk out on your partner when there's no legal and/or moral/community tie. ...
3) Because for some people, it matters deeply to them to be able to say that they're married. If that helps them get through a rough patch, who are we to deny it to them. Being in a long-term commited relationship is hard enough already.
My thoughts in response were:
True, marriage has been a contract for about as long as history cares to record. However, it seems to fit the definition given by <onelook.com>:noun: a custom that for a long time has been an important feature of some group or society (Example: "The institution of marriage")
As for the reasons why it should exist:
1) I suspect there are probably contractual arrangements that could be made to give at least as much security as a marriage, perhaps a living trust, for example. And while I'm not a lawyer or a family court wonk, I'd be willing to bet a sufficiently determined family can find ways to screw over a surviving spouse, too – maybe even get the marriage annulled. And frankly, if marriage is "only" a contract, it becomes vulnerable to the many and varied mechanisms lawyers have come up with to break other contractual arrangements.
2 (and some of 3) Why should a concrete reminder of a commitment need to be granted any legal status? You could have a marriage ceremony (or a hand-fasting, or any other type of ceremony) in front of your friends, your co-religionists, or in front of any segment of the community you like, without involving the legal system.
And frankly, if you want something to make it harder to just walk away from a relationship, why not draw up a contract, and have both (or all) parties put up a performance bond? The notion that if you want out, you have to forfeit $10,000 (or $100,000 or $1 Million) would provide a lot of incentive to stay together.
3) For some people, it matters deeply to be able to say they're married. Not "under contract", not "in a civil union", not even "committed". "Married". This term implies a special status, indeed a constellation of special statuses, not covered by any purely legal or social structure.
Marriage, as you point out, means a lot more than mere contracts even to the legal system. (Perhaps something like, "what God has joined together, let no man put asunder.")
Historically, as you point out, marriage has been a bond strong enough to override enmities between clans, tribes, nations, and religions. To me, that seems to go demonstrate influence beyond any mere political tool.
Indeed, you could consider it an institution.
And given the meaning marriage self-evidently has, it seems to me one makes changes in it only with great care, lest one dilute or completely eliminate those aspects of it that give it its power.
But the question I raised remains.
What is the function of marriage, and what makes it any different from any other arrangement available in society? Why does it need special rules, a special status, or anything else that differentiates it from other arrangement?