For most situations, I have the atitude that I should just mind my own business and not try to interfere with sexual decisions mutually made by consenting adults.
For some polygamous marriages the problem is, how can one know that all parties concerned entered the arrangement of their own free wills? Some of the polygamous marriages that happen in the Fundamentalist Mormon offshoot of the Church of Jesus Christ of Latter Day Saints involve underage females who are given as a kind of "prize" to some older high ranking male members of the church who already have several wives and some of the young women are coerced into this arrangement. One example was a sixteen year old girl who was severely beaten by her father for not agreeing to marry his brother, her uncle, and she ran away. That's just so wrong on so many levels that it makes me sick.
I know in some African countries, the Moslem men take on more wives despite the objections of their first wives, because they can't even afford to support their children with the first wife, let alone support the children of the second and third wives, so then all the mothers of the children are left to try to support their children alone.