Stats tell the story pretty well. 2/3 of people married in urban areas get divorced, and a marriage has about a 50-55% chance or reaching 25yrs. 82% are initiated by women.
80% say they would cheat if they wouldn't get caught. Let that sink in. 80%. Estimated cheating rates are 25%-75%.
Does that sound like a system that is "working"? Presumably not. Monogamy worked pretty well 50+ yrs ago when we had a very patricarchical society. In present society? Not so much.
I think those stats make it clear that monogamy is not a natural state for humans. While essential for the rise of civilation, it's definitely against our biology, which was thought to be serially monogamous (woman is with a man for 3-5yrs to get a child to where it's more self dependent and less need for a man, then move on to next guy-> good because more sexual variation than lifelong single partner).
Also explains why women are biologically set up to get sexually bored of a guy after 3-7 years. There is a reason the "post nuptial shutoff" thread exists, and it's not all from crappy husbands and stress.
Monogamy does work in some cases. Will of course depend on the mindset of the two people and their level of commitment and understanding of what will happen. Usually good if both partners have busy/robust careers and/or a low sex drive.
That in mind, knowing the stats I can't recommend monogamy. I can't recommend polygamy (multiple relationships) either, very few humans can love multiple people and keep that under control. What does that leave? Open relationship/marriage. One long term pair bonded relationship with emotional investment. Then one other girl (could be more, but one is plenty) who is a pure FB. Sex with her at least once a month. More frequent outside sex or more than one FB would depend on your sexual desire and being a high sex drive guy.
Obviously, woman is free to have FB type relationship with other guys. Some women will on and off, others never will. Horrible, biased system if a guy tries to say "I can have sex with whoever but you have to be exclusive to me".
It's a solid system, kids are possible, still get pair bonded deep relationship.