Rape is illegal. Since males can't force females to procreate, female choice is the deciding agent in determining who mates with who. Females choose mates based on their genetic fitness and their ability to provide resources. Genetic fitness is determined by all of: a male's ability to acquire resources (wealth), his looks (square jaw), talents (guitar player), personality (funny), social status (fame/popularity).

If a female no longer requires resource provision because she is earning enough money, she will prioritize genetic fitness in her mate selection, and so will be MORE willing to engage in a polygamous arrangement because polygamy can get her access to a greater diversity and quality of genetic material. Monogamy is only necessary for the female if it will provide her with resources or protection. Monogamy is not necessary for her to get access to sperm--sperm is cheap. She doesn't worry about money and can instead get the best genetic material without the best resources.

Therefore, female wealth drives polygamy.

But male economic inequality also drives polygamy.

Both factors increase polygamy. The only way to reverse these pressures would be to legalize rape and return to a state of nature. This would dethrone female agency as the factor that determines who will reproduce with who, and would elevate male violence to the deciding factor.