As the US continues it's "national conversation" on same sex marriage, it's fairly standard for someone to suggest that it's time for the state to get out of the marriage business and have marriage be a strictly religious/personal arrangement. This seems like a fairly neat way to sidestep the issue of having to reach a state consensus on what marriage is, with the inevitable one-side-tramples-the-other problem that suggests. However, I'd like to suggest that it's an impractical and illusory solution.
To start with, I think we need to look at why the state is involved in marriage in the first place. I'd suggest that the reason has little to do with managing morals or family values, it has to do with the essential function of government: being an arbiter in disputes, primarily about property. In this regard the state ends up needing to define marriage and know who is married in order to answer two questions: who owns what and whose kids are whose.
Say two people have been spending a lot of time together for the last five years. Now they've had an argument and want to not see each other again, but one of them claims that some things in the possession of the other are actually his. Are they? The state gets pulled into these questions because its job is to arbitrate disputes rather than leaving people to solve them the old fashioned way (which was by raising themselves up on their hind legs and bashing each over the head with flint axes.)
Marriage is a relationship in which property and resources are shared, and so when one breaks up the state ends up negotiating the breakup. People who are not married own the things that they themselves hold title to, and if they have a fight and stop seeing each other, the that shrugs its shoulders. My wife can claim title to a portion of the car or the house even if it is my name that appears on the deeds. My friend cannot.
Since the state also likes to tax people and make sure that they're taking care of their dependents, it also needs to know who is married in order to know who can claim to be a household for the purposes of taxes, child custody, etc.
For this reason, any state which isn't borderline anarchic is going to have to know who forms a household, which traditionally has meant knowing who is married. Despite this, "civil marriage" is relatively new. In the past, figuring out who was married was pretty easy: someone went down to the church register and checked to see if a couple were married, or you just asked the neighbors, "Were these people married?"
As cultural consensus has broken down and society has become more mobile and impersonal (and also as the state sought to replace religion and culture as the primary context of people's existence) civil marriage was instituted to give the state a clear and easy way of knowing who formed a household: If you were married, yes. If not, no.
And here is where we run into the problem: In our modern world, living arrangements have become more complex than ever. Almost everyone has enough property to fight over -- as anyone who's ever been stuck watching Judge Judy in a waiting room knows -- and due to changing morals and the affluence that allows people to turn their attention away from the basic "survive and produce the next generation" household structures which have characterized most prior societies. (For instance, despite all the talk about homosexuality in the Classical world, even men who preferred their own sex were expected to marry and produce children, whatever their extracurricular activities. In our modern world, this expectation no longer exists.)
The state cannot duck the situation, because arbitrating property disputes is one of its most basic purposes, and determining who constitutes a household is one of the basic elements of resolving property disputes. And yet, any modification to the definition of marriage/household as recognized by the state is necessarily fraught with cultural and moral significance because it effectively means changing the definition of what a recognized or proper family is.