Yeah, we would have to completely rethink so much that I don't think it's worth it. I mean think about any legal difficulty or ambiguity you can, now add complications like paternity rights for children.

Since me and my wife got married a decade ago, we've had 5 girlfriends and a boyfriend between the two of us. Two of the girlfriends ended up being serious longer relationships... and I still don't think the effort that would go into setting up the legal architecture is worth it.

Frankly long term relationships with more then 2 people rarely happen of their own volition. When they do it can be super intense and really beautiful, but I think most people who are actually in those relationships are willing to accept the reality that legal recognition in the form of marriage isn't likely to happen. We certainly where/are.

Although it would be nice to be able to have more flexibility when immigrating. Marriage gives you tons of added status there, and having a relationship with more then 2 people when your trying to move to another country becomes a big issue.

Usually relationships like this are well worth any trouble we've personally encountered or can envision though. No one says you have to get married to do so. Nothing requires marriage be a part of your romantic relationships in the first place, and in many countries a good lawyer can help making sure non-married partners have most if not all of the rights afforded by the law to married couples.