Monogamy is overall better for society, kids, and most people, especially women, find it more satisfying most of the time.
The thing is, there s no perfect arrangement or system that everyone will feel happy with 100% of the time. Even just looking at one individual, people often have conflicting desires, or act in ways that later make them miserable. Many people do things they know to be bad for them, bad for those around them, either occasionally, or a lot.
I fairly regularly eat crisps which is not only bad for my plan to lose 10 lbs, it causes me pain a few hours later. I know it will be bad for me, but I do it anyway, because I am weak about salt cravings, and crisps in particular.
No one would suggest that I just forget about a healthy diet or that it's silly to worry about the pain later on. Who knows, if I didn't at least try to resist I might way 300 lbs and need abdominal surgery for whatever the heck is wrong in my guts that causes the pain.
The people who really do best in a society without monogamy are hot, rich, or powerful men.