I honestly don't think that relationships with more than two people in (and I'm not talking about exclusive "one man, two or three women", there's also a thing called polyandry) are inherently bound to be oppressive. We've seen literally thousands of "normal" monogamous relationships that were just going awful.
Yup, you could use the exact same arguments in this thread ("but we live in a patriarichal society", "look at all the abuse that happens") to say that we should ban
all marriage. In a similar vein, the specific problems that emerge frequently with polygamy are generally things that are already illegal. When a psycho in Utah marries his two teenaged nieces, the least important part of that is that there's two of them, as opposed to his committing incest and marrying underage girls.
Well that's pretty much the crux of what social liberalism is today, which is why I stopped calling myself one a couple years ago despite coming down on their side of most of the mainstream hot button issues. Pretty much the same mindset as social conservatives, only different values being imposed. Despite all the talk about multiculturalism, very few people truly represent pluralism anymore