A toaster cannot consent. Why is polygamy illegal if all parties involved consent?
The reality is that it can't be illegal. Marriage has been redefined, and the new definition isn't broad enough. We're stuck at two consenting adults right now, there is no reason it can't be 4, 8, or even 16.
The state either recognizes or doesn't recognize contracts between individual parties.
So, is the marriage contract including 3 individuals, or are there two separate marriage contracts including 3 individuals? To the state that does the recognizing, this could/would be a key factor.
Personally, I don't give a shit one way or the other, but I'd think that a state has an interest in not recognizing polygamy if there are two separate contracts, because you start getting into all sorts of tax and rights issues where the state literally has to choose one marriage contract over the other. Maybe it recognizes the first one and nullifies the newer one as void, or maybe it recongizes the newer one as voiding the older one, much as newer wills and codicils can modify or even nullify the older will.
A single marriage contract made between 3 people would be a lot easier for a state to recognize given most tax and rights laws, as all three individuals would/could be considered married to both other parties, although there would still need to be a lot of changes made to current laws to clarify what happens for taxes, births, deaths, dissolutions of the marriage contract, etc.
But, what is clear, is that allowing gay marriage in and of itself doesn't mean jack shit in terms of polygamy, as gay marriage is just like straight marriage, except that any two individuals who consent can enter into the contract, whereas before it had to be two individuals of the opposite sex. Or, to put it another way, existing laws don't really need to be changed to allow for gay marriage, but would need extensive changes for polygamy made up of one or two separate marriage contracts.
If you're for polygamy, first decide whether one contract or two contracts or more can be recognized, and then get out a long sheet of paper to figure out all of the laws that would require changing to make polygamy work with regards to all of the other laws that come into play with marriage.
Toasters, consent, straight marriage, gay marriage, and polygamy are all vastly different concepts in regards to marriage, because marriage is basically just a contract that establishes tax laws and rights for the individuals entering into the contract.