Earlier quoted context omitted.
How could it be consensual between all adults? The marriage "license" in polygamy is between man and women, yet by secondary effects the women are fully entangled with one another. Yet if you watch any show on polygamy you'll see that a guy will keep adding new, younger wives while holding their current wives hostage (due to their lack of another option in terms of housing, money, etc). It's absurd to think that any…
>How could it be consensual between all adults? Quite literally, I imagine it would go something like: Do you take this woman to be your wife? I do. Do you take this woman to be your husband's second wife? I do. Do you take this man and woman to be your husband and second wife? I do. That's how three adults could all consent. You can argue that someone is being forced to say yes, but you could argue that in any type…
That said all of the legal agreements become far more complicated and likely null and void if one person drops out or is added. I think an argument could be made, but with the complication you're basically saying polymarriages would be like corporations, with their associated complexity.
Polygamy as practiced now is a "bring more childbearing women" into the flock type thing, which is by nature non-equitable. I doubt most folks in a polygamous marriage (by choice) would want to get involved in a polyamorous marriage and as such, I think you can't legally support a non-equal union of that sort.