Jennifer Morse has a series of articles about why this won't actually work here, here, and here.

Here are some quotes that should give you an idea of her point.

Disputes that arise between the contracting parties must be resolved by an overarching legal authority. Let’s face it: that overarching legal authority always will be some agency of the government. “Getting the government out of the marriage business” amounts to refusing to define marriage on the front end. But the state will end up being involved in defining what counts as a valid marriage or parenting contract, on the back end, as it resolves disputes. We cannot escape this kind of state involvement.

No-fault divorce provides an analogy. No-fault divorce allows one party to end the marriage bond for any reason or no reason. In effect, the state redefined marriage by removing the presumption of permanence. Marriage became a temporary arrangement rather than a permanent union of a man and a woman. No-fault divorce was supposed to increase personal freedom.

and

[W]e libertarians have been starting our theorizing from the perspective of adults who are equipped to take care of themselves, make contracts, keep promises, defend their own property, and respect other people’s property.

But no one enters the world that way: we enter the world as helpless infants. In fact, if you think about it, infancy is the only truly universal human experience. We all have to pass through infancy to get anywhere else. Yet, we libertarians essentially explain the transition from infancy to adulthood by saying, “Then a miracle happens.”

Personally, I think we need to be more explicit about this step.