So all three of them are in a marriage together? And if one person leaves, the whole marriage is dissolved and the other two have to get remarried?
What if John wants to be married to Jane and Sue, but Jane and Sue only want to be married to John and not to each other? Can't discriminate against them, after all, so I guess we'd have to allow that. If John and Sue buy a car, what percentage of it belongs to Jane? Is it solely John and Sue's private property, even though John is also married to Jane? Does Jane own 25% of it, since John owns 50% and they're married?
What if Jimmy, Ralph, and Christina (a three-person mutual marriage) want to add Bill to the group, but he's already married to Christina separately? Do they all have to get divorced and then remarried? Or can he just be slurped into the marriage as a whole? What happens to Christina and Bill's joint assets when that happens? Do they remain Christina and Bill's, or do they become marital assets of the entire group? What about custody of Bill's child with Lisa, his other wife. Do Jimmy, Ralph, and Christina automatically become responsible for his care? Or does that matter stay between Bill and Lisa?
I don't know how you can ask why it would have to be obscenely complex with a straight face.