Now that we've solved the age old problem of people getting married way, way, way too young, we need to start on the problem of people clinging to monogamy as some sort of universal truth.
If it was a problem, why did so many societies choose that path back then? Lower overall life expectancy and the prevalence of disease required you to play a game of probabilities with reproduction.
Say you want four adult children. According to one source, 50% of children died before adulthood back 500 years ago [1]. So, you must plan to have eight children. Is this task doable if you start in your late 20's/early 30's? Likely not.
Also, life expectancy for the mother was far lower than today [2]. The mother will be providing the food to nurture the infants, so her early demise is to be avoided. Starting earlier is again preferred.
Modern society sets up different incentives for the marriage equation. We're told careers are more important than starting families.
Monogamy is not the only path to fulfillment, but people must seek the other paths on their own. Normalizing non-monogamous relationships for the unprepared leads to disastrous consequences for all parties involved.
[1] https://ourworldindata.org/child-mortality-in-the-past
[2] https://www.sarahwoodbury.com/life-expectancy-in-the-middle-...
Monogamy is a breeding ground for drama. It creates a lifelong conflict between choosing to be a faithful spouse, and choosing to follow natural human instincts.
I know a woman with an open relationship, she regularly has many partners but has a husband. A friend had approached her one time thinking she was going to make a difficult and shocking revelation to her: that she had seen her husband with another woman. It was then that she had to delicately explain the arrangement of her open marriage to her friend.