There is no guarantee that a guy would stay and form a family, especially in these modern times.
Look, the more primitive a civilization is, the more patriarchal it tends to be. And this is exactly what "progressives" want to get away from.
There could be a contract. Ideally, one drawn up by the individuals involved in the compact, not one handed down by the state. I think the current deal offered by the state has serious shortcomings, but that's another tangent.
I do hope you realize that women are perfectly capable of breaking familial commitments as well.
If they want to get away from the time-proven solutions, that's fine, but they have to come up with an alternative on their own. Passing the cost onto society should not be among the options to consider.