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.).