Marriage and divorce is really a side issue. As a citizen, I have no interest whatsoever in the details of the relationship of any couple. The duration and conduct of their relationship (excluding behavior which would be criminal in any context) simply does not affect me in any meaningful sense. Get married, get divorced, live together, homosexual, heterosexual, missionary position with the lights out, whipped cream, whips & chains, leather and latex, it is all one to me. Divorce per se does not threaten or diminish our society at all.
As a citizen, I do, however, have an interest in and a responsibility to the well-being of children. The parents naturally have the primary interest and responsibility, but unfortunately not all parents discharge their responsibility with even basic competence. We have a seeming consensus that parents are not given carte blanche in their treatment of their children; they are required to answer to their community for their basic conduct as parents: health, safety, nutrition, etc.
Tying the care of children to the details of two people’s sexual relationship seems without strong rational basis. It seems much more logical to allow (or require) that two (or more) people to enter into a civil contract to provide for the physical, economic and emotional care of their children. Naturally such a contract and its governing law must be carefully constructed by those with greater legal and political skills than mine. Still it seems we have quite a lot of experience in understanding contractual obligations, sufficient to develop reasonable strictures and defaults.
The details of two (or more) people’s sexual relationship and specific lifestyle really cannot form a legitimate basis for any sort of legal or governmental action (again, excluding otherwise criminal behavior). I see no reason why we should not focus our attention on securing and enforcing the well-being of our future citizens rather than on the trivial and private details of people’s romantic relationships.