Those aren't business arrangements. At all. Your pension contributions aren't calculated on the number of wives you have. Survivorship pension premiums for spouses aren't calculated for an arbitrary number of wives that can change at any time.

Doctors ask for medical decisions from from an arbitrary number of spouses.

You're not asking for something that "requires some thinking". You're asking to abolish the standard marriage contract and demanding that everyone get a customized marriage contract. How the hell is anybody supposed to know if you have a standard 2 person marriage or are a polygamist? If you're a polygamist, then you bitch about being discriminated against with high cost policies and exclusions that are reserved for 2 people marriages.

That's not feasible, and infringes on other people's rights and adds exorbitant costs to insurance policies, medical policies, etc.

The overwhelming majority of people don't waste money on lawyers hammering out prenups and divorce clauses, like uber-wealthy celebrities do.

I don't expect to change your mind on this, because people (i.e. men) who are dead set on this aren't receptive to reason. Or even fucking grown ups with responsibilities. Your delusional little harem idea is just that, a delusion. That's why the overwhelming majority of these kinds of arrangements are done to brainwashed people in cults, instead of fully informed grown adults.