Genuinely curious, if you want an open and polyamorous relationship with a group of people, why wouldn't a structure similar to an LLC or similar business group not be roughly sufficient (as far as a framework goes)?
It starts to break down for child custody as well as medical directives, end-of-life decisions, hospital visitation. You also don’t get marriage related tax benefits or the ability to hold title to real estate or financial accounts as “tenants in the entirety” (which is permitted only in some states, solely for married couples, that protects joint assets when only one person has claims against them), which is important depending on the creditor laws of your state and how they handle claims against a member of a multi member LLC.
People can use different terms to distinguish between different things, but that doesn't necessarily make any of those things "second class," full stop.
All label the upper/dominant class uses to show they are giving those below them everything they are entitled to, except actual equality, while firmly keeping them where they belong.