Late to the discussion, but I think this is really something that is a tricky issue. It should be simple, but it isn't.
In a perfect world, people who wanted the legal obligations of marriage would enter into that contract. And those that didn't wouldn't. People would look to protect their own interests and make decisions accordingly.
The difficulty is that the interests of the parties involved conflict, that is the point of creating the legal obligation. To protect the more vulnerable person, usually the woman who has had a child.
Unfortunately what happens is the person who is most vulnerable to exploitation may also not be in a position to set the terms of the relationship and insist on marriage. And that is more true, the more vulnerable she is.
It's especially so now that there is little or no social pressure for the couple to marry, the woman who would potentially be protected in these cases can be made to feel she is being pushy, and has no leg to stand on to insist on marriage as being important.
The other side of that, for me, is that the woman who does not want to marry in order to protect her assets is, in many cases, fairly well off, and actually in a much better position to negotiate, have a pre-nuptial agreement, or protect her assets in some other way. Including by not cohabiting, since she can likely afford to live alone.
So on the one hand my gut feeling is that people should be able to enter into the contractual arrangements they want. But realistically, people pair up and that is still often a mainly economic decision (as marriage always has been), and it's important to have social structures that protect the more vulnerable members of such a union from exploitation. And if it's socially and legally something they can opt out of, then it won't work.