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 of marriage or contract.
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.
Hmmm, please excuse my ignorance in such matters. I only married one woman and that is quite enough.
>you're basically saying polymarriages would be like corporations
Yes, exactly. Well put.
>with their associated complexity
I can't even begin to imagine the complexity, legal or relational. However, I can imagine situations where such an arrangement could be beneficial to the individuals involved.
Are you trying to say multiple adults can't all agree on something? Your argument is based on ignorance and you're no better than the people who oppose gay marriage. You've been fed Christian and feminist propaganda to make you believe that only two consenting adults can love each other and that if you happen to love more than one person you shouldn't have the same rights as other people who don't.
You basically don't think polygamists are equal. Now let's see you do a full 360 from being pro-equality to using the same arguments the bigots used against you.
Smart folks have said that the reason homosexual marriage even became a thought was because of the transformation of marriage from a bond of ownership of man over woman to a shared partnership. A polygamous arrangement (as per the mormon church, not the polyamorous relationships of the hippies, etc) ends up with a hierarchy of power with one man controlling multiple wives, with the power to divorce, ruin, etc the others if they don't allow him to wed again.
I think an argument for polyamorous marriage could be made, but polygamy is essentially a raw deal for whoever isn't the hub of the marriage wheel (one man in the case of traditional mormonism, islam, etc.).