Until they were purged by the Medieval Catholic Church, many western cultures had marriage alternatives -- legal, contractual, long-term relationships within which children could be reared but which were not until-death-do-us-part marriage. Ironically, Church father Augustine had a child in just such a relationship.** Reading articles like this one, it strikes me that it is time for some modern innovation here.
We are in a position where we have just two alternatives -- marriage, which is a full-blown legal merger of two people into one for what is theoretically life -- and nothing. Given the rise of childbearing in these no-long-term-commitment-whatsoever relationships, the state has taken a few halting actions to bridge the gap, but most of these have been ham-fisted and fraught with problems (our efforts to impose financial responsibility on fathers is one example).
If this were a market, I would say that there is clearly a consumer demand for an alternative product that fits between marriage and nothing, and allows two people to make long-term commitments to child rearing without necessarily commingling assets or making lifetime sexual monogamy vows.
** At the time, such concubinage relationships were often to satisfy class issues -- people of certain classes simply were not allowed to marry each other. In Augustine's case, it allowed him to pursue a 15-year relationship with a woman who was not wealthy but left him available to marry when a rich woman later came along. In short, it was used for reasons that are mostly irrelevant today. But that doesn't mean we can't invent marriage alternatives of our own for our own modern reasons.