Exactly. I think the libertarian way to go would be to limit the role of the Govt. to enable Tax breaks for spouses, and other impersonal things.
"Getting married" is very easy and requires almost no expense and only a little bit of time. You don't need to plan at all for a civil marriage, just sign some paperwork. Drafting all the documents you'll need (will, medical directive, etc.) without a marriage takes lots of time and money (lawyer fees). Writing marriage into laws is a way to save families time and money and make everything simple for them.