The reason monogamy is ideal is because human beings are a tournament species[1] that's smart enough to have realized that emulating a pair-bonding species results in better societal results. Mainly because if we just followed our animal natures a huge percentage of men (far greater than now) would be shut out of the mating market. Having a large population of young males with no mating prospects hanging around is harmful to society for many reasons, the most severe being one of the charismatic winner types deciding he wants to sell them a dream and putting them to nasty work.

[1] https://www.uh.edu/engines/epi674.htm