You have a very narrow definition of marriage, relationship, and commitment. That's ok, many people do. Your error is in assuming that your view is the "correct" one and thus defining anything falling outside of that as wrong. And the sad part is that such a restrictive definition means that human nature and desires will always constantly be on a collision course with that definition of what is acceptable. Many defenders of mandatory monogamy will unironically say, "sex is not the most important thing in a relationship, it's about love and commitment" while also saying, "sexual fidelity is so important that one breach can invalidate decades of relationship." That is inherently contradictory.
Monogamy, as a cultural concept, is derived from questions of property ownership, combined with the belief that women were owned by their parents and ownership was transferred to their husband. It then evolved into some sort of sense of the moral superiority of being with only one person at a time. If you have a little time (~35 minutes), this conversation is pretty enlightening: advice columnist Dan Savage talking with Esther Perel, author of “Mating in Captivity.” Dan and Esther take long hard look at the history of human relationships and our culture’s obsession with monogamy.
https://overcast.fm/+GeYbxRhmo/45:45
An excerpt:
Savage: Meg Barker writes that the pressure to perfectly execute monogamy over the lifetime of a marriage, half a century or more, makes every monogamous relationship a disaster waiting to happen. [...] Monogamy is the only standard for success. You did it perfectly or you are terrible at it. I get in trouble for telling people that if you're with someone for 50 years and they only cheated on you once or twice or three times, they were good at monogamy, not bad at monogamy.
Perel: For most of history, monogamy had nothing to do with love, and was primarily an imposition on women. Monogamy has never been an equal gender proposition. Men practically have a license to cheat, they have a whole series of theories that justify why they are natural roamers. Women were created as a domestic creature that never wants to go anywhere -- and I don't understand why she gets locked up everywhere if she never wants to go anywhere, but that's another thing. Monogamy was basically an economic imposition in order to know whose children are and who gets the cows when I die. It's about patrimony and lineage. Then it was one person for life. Then it moved to one person at a time. A woman told me: "I was monogamous in my two marriages and with my three boyfriends since." I am monogamous in all my relationships.
Savage: Serial monogamy.
Perel: The concept changed, that it's one person at a time, not one person for life. When we used to marry and have sex for the first time, monogamy meant one thing. But today you marry and stop having sex with others, monogamy means something else. Exclusivity, coming after 10 years of sexual nomadism is very different from exclusivity that comes from coming as a virgin to marriage and then having your first and only experience for life. These terms are fluid.
I would also recommend reading this article, which explores the ideas that while monogamy may be great for some people, some level of "infidelity" may actually be what helps some relationships survive where sexual needs may differ or change over time:
Married, With Infidelities (Published 2011).
In short, monogamy may work for you, and if so, great! But it may not work for everyone, and defining it as the ONLY acceptable form of relationship makes it an impediment that, for a lot of people, will cause them to always be on the wrong side of an unrealistic standard and thus always feel like they're failing at something that isn't right for them.