The third party that either enforces, or challenges contracts is the party that "gives a shit." Both common law and state certified marriages give rights that are only valid during times of outside influence, be it a challenge on inheritance by a wayward family member, or a challenge on a request for benefits by the government or an insurance agency. That's where a third party, be it a private entity or the government, comes in to enforce or better define the contract. So, if you and your partner are consenting, and have a contract between the two of you, that contract will only go as far as keeping the two of you in agreement over the terms. If one of you dies or becomes incapacitated, you are still strangers and have no contract in the eyes of the people and groups that actually matter :/ I think the only question remains is who do you want to entrust and enforce that contract through: government or a private entity.
P.S. not trying to be soapboxy, either. You have points and facts I had not considered and was not aware of, am glad to learn these from you, and appreciate being able to bounce my ideas off of your brain as it were.