See my other post about the original point of sanctioning marriage.
Just because the state feels like it belongs in the marriage business doesn't make the state right. Removing that power from the state doesn't require anarchy, it requires people to tell the state to get the fuck out of their personal lives.
Point taken but given that marriage is a legal concept I don't think the state will walk away from it. And nor should it. The whole idea of marriage is to give legitimacy to a union. The problem is that the union is - for the purposes of this discussion - un-necessarily restrictive. It's not so much the state butting out as the state needing to recognise that the "traditional" model may not be the sole definition of marriage any longer.
Just because it is a contract doesn't mean the state needs to sanction it. I can enter into lots of contracts without the state sanctioning them. For example, I have a neighbor that mows my yard. I pay him $25 for that. It is a verbal contract that him and I have agreed to. the state didn't sanction it, has nothing to do with it and cannot tell either of us that we can or cannot do it. It is a legal contract for all intents and purposes. If I pay him $25 and he doesn't mow my yard, I can legally sue him. On the other side, if he mows my yard and I don't pay him, he can sue me. It doesn't require the state to acknowledge it or sanction it. We have court systems for contract violation, we don't need the state to sign off on every contract.