The OP really needs to clarify their position. The question is simply too broad to answer adequately. Certainly in historical context polygamous relationships have allowed abuses of power, but then so have monogamous ones. In and of itself there is no reason that a balanced relationship should be any more ethically suspect simply because it involves multiple partners.
Cindy is Married to Frank and Mark. Everyone contributes to the household in some manner, and While Frank provides the bulk of emotional support to Cindy in just the ways she needs, his sex drive is very low and was a frustrating factor in their relationship. Mark is in many ways, Frank’s opposite. He is charming and a pleasant man, but is not really there for Cindy in the manner that Frank is. He does however, provide frequent physical affection for Cindy and is a great friend and occasional lover to Frank who is bi-sexual. Collectively, they have 3 children, and due to the fact that Mark travels often for work, Frank is able to be a stay-at home Dad and telecommute.
In this case, everyone benefits from the arrangement; If you take the basis of ethics to be that the creation of harm or suffering is “wrong”, then this relationship is not ethically suspect. You could create any number of other scenarios With any combination of genders and reasons and get the same results. Our current objections are more a creation of historical bad memories and religious baggage than anything else.