Here’s an argument that the state should only affirm, and not regulate, the ways in which people conduct their long-term private relationships.
If I leave aside for the moment the religious dimension of it, and focus simply on the distribution of state benefits, I can foresee the following problems. When does a partnership become one that carries with it survivor benefits, such as inheritance and social security benefits? Are children entitled to survivor benefits from anyone other than their natural or legal adoptive parents? Is there any basis for restricting a person to only one partner at a time? If not, what happens to a social security system in which someone can obligate it to pay out survivor benefits to an essentially unlimited number of partners? Or do two spouses simply get half the benefit one spouse would get, three spouses a third, and so on? Such treatment flies in the face of our commitment to fairness, and wouldn’t long persist, I suspect. But paying out multiple survivor benefits would likely require either a diminution of benefits overall or a substantial increase in social security taxes. Or perhaps it would lead to an attachment of benefits to individuals, with nothing of consequence following from a relationship. The family--any sort of family--would lose its "privileged" status when it comes to certain sorts of government benefits.
In other words, government support for, or acquiesence in, the free choices of individuals could in the long run lead to the elimination of government support for the family. This can’t be good for children, but, then, who cares about them?