What do you mean by _marriage_? If it's just some kind of a contract, and one believes in equal access to the rights of association, any instance of it between informed, consenting, competent parties would seem to be okay. (In a liberal context, anyway.)
As I was once told some French writer wrote, "The burdens of marriage are such that they are far too great for two -- and can scarcely be borne by three!" I can't find it on Google, though.
The problem with many forms of polygamy is that in fact it's polygyny based on women being assigned an inferior status (and thus are not really consenting).
-- Gordon