Hrm... not sure such a bias makes sense, especially in a group marriage where everyone was involved in raising the kids, for example. (Again though, I'm not saying these things are unsolvable. Just... there's a bunch of little things like that that would, in fact, have to be specified.)
(I'll avoid commenting here on the extent to which the system presently mucks things up, since that's outside the scope of the discussion.)
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