It's interesting that many liberals have been ecstatic about same-sex marriage being passed; but if I were to ask them about polygamy, they would instantly turn around and condemn opening up marriage to such relationships. This is not to say that gay marriage and polygamy are the same. They are different and there are different considerations required for each. Yet, even liberals use the same arguments used to oppose gay marriage against polygamy as if they would somehow be successful for one and not the other.
Why not polygamy? The arguments against it are pretty standard, but let's go through them one at a time.
