aprilsimnel and Darwin bring up a very good point that could be a powerful focus in your paper. Younger men are thrown out of those societies so that the more powerful men can choose even more wives for themselves. Sometimes these boys are extremely young when kicked out and have no idea about the outside world. They are left to fend for themselves and do not have the faintest idea where to go for help.
These societies are found all over the world, including America.
Another negative aspect you can focus on is that women typically have no say in who they marry, at what age they marry, etc. Sometimes females marry relatives, and sometimes very young women marry a man who is old enough to be their father or even grandfather. While these societies are not strictly prohibited in the United States, child abuse laws still apply to them. If a 13-year-old marries a man old enough to be her father, she is obligated to have sex with him the second she has her period – you can see how this breaks the law.
(I’m taking cultural anthropology and there’s a lot of information about things like this.)