'No woman would want to be in a harem willingly. They all want to monopoly the man and eliminate other women."
That's just not true, and there's a myriad of reasons why a woman would want to be in a harem willingly, as history has proven.
Historically, men are more expendable biologically because they can produce a lot more sperm at a much lower investment than women can eggs, therefore they can have more kids across more people with less cost to themselves as opposed to being pregnant. Females have to be much more selective about their mates because it is a greater risk towards themselves and so will seek more means of acquiring better resource both for themselves and their children, and so males in nature will have to prove themselves by competition and acquiring more resources which is more potentially dangerous.
Hence why in our ancient history, less Males produced more kids with more woman, which is consistent with our evolutionary relatives such as Chimpanzees, Orangutans, and Bonobos.
Comparing it to more recent history, marriage has been more of a political custom, and when one man is particularly successful, he tends to acquire a harem because he has the means to provide security, resources, and fame, and so it attracts women to them as it ensures them the best quality of life, and ensures that their family is held in high regard. It's not all sunshine and roses, naturally, and harems tend to be filled with infighting as each person tries to get on top and have their bloodline spread, but it does not mean that women wouldn't naturally want to include themselves in a harem if the need arise. It's less preferable, but it's just as probable.
It's really an economic argument. People ideally would monopolize their mates but that rarely is certain to happen nature so some may choose to share but try to get more of the resource for themselves. It's too broad to conclude that all women would not be happy, or would want to monopolize a man just because that's the standard in most western countries and westernized countries.
@miyako19