You're thinking of polyamory. Polygamy refers to one man marrying multiple women, i.e. a structurally unequal marital relationship.
Well, I was thinking of marriage. The question was if it's defined in the style of the Middle East where a man can have a harem with obeying women or if it would be a marital version of polyamory. The latter doesn't exist in legal form in first world countries, but hypothetically, if it would become legal, would it be morally acceptable. I'd say yes in that case, but if polygamy exclusively concerns the former definition, then no.