Actually, you answered the question yourself.
The fact that polygamous societies are hellholes for women and children is good enough reason to say polygamy is bad.
If a moral reason isn't good enough for someone, then the practical outworkings of such a society ought to be enough to convince someone. But sadly, we're seeing that isn't even enough for some people.
Hey, I’m with you. I think polygamy would be awful. My point was that people who concentrate on the political and philosophical arguments have their minds clouded with the world of ideas, while here on the ground, real people suffer.