I have never understood why it is illegal to have more than one spouse (this comming from a super conservative, right wing, christian background). As long as all parties are in agreement why is there a problem. I guess it would be handled like any other legal partnership. All must be in agreement to add a new partner and the terms and percentage involvement is determined in the beginning, but can also be changed and altered at a later date.
Benefits are another topic I have always had an issue with, why can't we decide who is to be covered under under health insurance and retirement beneifts, especially if they are part of the same household.