Polygamy is a bad thing because it concentrates power, decreases equality for women and increases resentment by lowers status men, who have little hope of a hap

Polygamy is a bad thing because it concentrates power, decreases equality for women and increases resentment by lowers status men, who have little hope of a happy life. Usually polygamy only create a small set of legitimate heirs and a larger subset of 'bastards' who also have little prospects in life within these societies. Polygamy is actually a rarity in human societies, and while it definitely exists as a cultural practice, it's usually in a society with heavy restrictions on freedom. What is more usual is monogamous relationships with a small amount of adultery on the side. Given the choice of the two, the latter is arguably a better choice for societies, which is why it is the norm.