It also included so-called 'concubines,' which is another word for random woman you are not married to. As long as these women weren't married to another man, the Bible doesn't seem to have an issue.
I forgot to mention concubines. If Solomon can have 700 wives and 300 concubines, how is that not an open marriage? But then again, the bible is full of laws showing women are just property like this one.
If a man encounters a young woman, a virgin who is not engaged, takes hold of her and rapes her, and they are discovered, the man who raped her is to give the young woman’s father fifty silver shekels, and she will become his wife because he violated her. He cannot divorce her as long as he lives. - Deuteronomy 22:28-29