All these things that I have done

I thought it was odd to end with a solution with person_1 and person_2 fields again. Here’s my suggestion for fully generalized civil unions. First, decree that such a household needs to have a name, agreed by the parties. In conventional marriage in the English-speaking world, the husband’s surname is used, but once you have a rolling union of polyams you may decide to choose a separate name for the marriage itself. Then things are simple enough: there’s a table union_members with a union_id, person_id, plus dates joined and left. This can admittedly represent zero- and one-person unions, but either you ban them at a higher level of business logic, or you accept that a person living on their own or as a sole parent is a marriage of one in some ways—and a zero-member union might be the legal husk of a marriage where all the members have left but the children or other legal obligations live on.

This is horribly similar to a made-up society in an sf story I never wrote.