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.
A thought occurred to me today as I skimmed an article in a rationality forum where the subject of gay marriage cropped up; seeing as the issue has been hotly contested in various public fora and especially the courts, what about poly? After all, many if not all the arguments for gay marriage apply to poly marriage as well.
Questions for LWers who are currently in a such a relationship, or have an opinion to share:
Do polies want to marry each other or do such relationships not lend themselves to permanence above a threshold of partners? Should polies campaign for the right for a civil union anyway? what are the up and down sides of this? etc