Well kind of difficult to elaborate on it given that I don't know what to think. But I'll try.
This is one of those topics I've never really thought about before, but my initial reaction when hearing that a polygamous family raises children would be a feeling of uneasiness. Having multiple mothers would complicate things for the child, there is much more potential for conflict within such a family. Moreover, parents would have to be very careful that their child doesn't get the wrong impression about the family (like thinking that there is some kind of inequality between men and women).
I realize this is quite a half hearted answer...but raising children is about the only thing I can think of when arguing against polygamy and this is a debate forum right?