Legal Marriage

Why?

I recognise I'm a bit late to the party on this one, but with all the discussion about the definition of marriage over the last year or so, why have so few people asked this question:

Why does the government regulate marriage at all?

It hardly seems like it's their business. I know we can make the whole thing voluntary by eliminating common-law marriages, and the liberal application of pre-nuptial agreements, but that still leaves the government administering a system that defines people's personal relationships, and I fail to see how that's their job.