Why is the state involved in marriage at all?

From Ed Morrissey at Hot Air, a question I've been asking for over a year now: Why is the state involved in marriage at all?

Really, if we didn't have marriage in this society, why would we have to invent it? Wouldn't some contractual arrangement work as well? Maybe a "Chapter M" corporation? Do we really need a specialized arrangement for sharing goodies between two people?

...Townhall’s David Harsanyi offers the argument that government involvement may do more harm than good to the institution, and results from a historical mistake in the first place. Time to get on with the divorce, Harsanyi insists:

In the 1500s, a pestering theologian instituted something called the Marriage Ordinance in Geneva, which made “state registration and church consecration” a dual requirement of matrimony.

We have yet to get over this mistake. But isn’t it about time we freed marriage from the state?

Imagine if government had no interest in the definition of marriage. Individuals could commit to each other, head to the local priest or rabbi or shaman — or no one at all — and enter into contractual agreements, call their blissful union whatever they felt it should be called and go about the business of their lives.

I don’t think that this is an easy path to adopt, it’s going to be the eventual solution. Not only does it take government out of people’s private lives, it also means an end to a divisive and essentially meaningless debate — and it protects houses of faith and ends a potential government interference in matters of religion.
....
We would do much better to require people to create partnership contracts in the civil context than get marriage licenses for issues like property sharing, access to family, and so on. If people want to live together and share their lives to that extent, it’s healthier and much less confusing later to have those issues expressly spelled out in an agreement up front, just like any prenuptial agreement today. If two people don’t want to go that far in formalizing their relationship, then they shouldn’t be considered married anyway — and shouldn’t get access to “palimony” and have debates over oral contracts, and so on. If you don’t get it in writing, it doesn’t exist, in the context of personal partnerships.

Then, if people want to get “married,” they can go to the institutions that actually care about marriage: churches, synagogues, mosques, temples, and so on. Marriage can be a private, faith-based recognition of a sacramental relationship that exists outside of the civil context entirely, and houses of faith can set their own requirements as to what it means and who can participate — just as they do now. Not only does that protect the sanctity of actual marriage much more than a government, but it also means that government has no way to poke the camel’s nose of intervention into the religious tent, as it were, to force houses of faith to conduct marriages that violate their tenets in the name of fairness. Divorcing marriage from the state and dissolving the partnership between government and religion benefits the latter more than the former.

Let government define and enforce contract law, not marriage. If we don’t follow that path, people will shortly become very unhappy about the eventual government definition.