Chuck ó No, polygamy was not an invention of Islam, and indeed is practiced by other cultures as well, even today.

And most of those cultures, I maintain, treat their women badly.

And are you equating the practice of keeping mistresses with polygamy? In what culture did mistresses, and more importantly, their children, have anything like the legal and social stature afforded to wives and legitimate heirs? Historically, bastardy has had many legal and social sanctions attached to it.