Agreed on all of this.

More generally, what I want laws governing marriage and parentage to do is formalize social support for family arrangements in a way that provides equal support for all families that share the properties that I value supporting. The most important of those properties is a commitment by an individual to provide logistical, economic, and psychological support to another individual in times of crisis.

Within the specific context of poly marriages, I want the law to do that for families that include more than two adults, but that's just a special case of what I think marriage and parentage law (more broadly, family law) is for.