This. Poly marriage is also more complicated because of child custody (in particular given the increased instability of a relationship the more people who are part of it). Also significant is that there are multiple kinds of poly arrangements and covering them all in a useful legal way is difficult.
I say this as someone generally supportive of poly relationships. Eventually it'll be a fight worth having, but I'd wait for the current struggles over gay marriage to die down (I give that about 20-30 years)
(in particular given the increased instability of a relationship the more people who are part of it)
Children by default stay in the marriage, unless the judge rules that the it would be best for the parent leaving the marriage to have full custody. Voila problem solved.
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