Don't you think some people can't do poly or monogamy because one is ingrained as "wrong" to them? It's not the same as sexual orientation but I feel most people navigate one better than the other. Besides there are so many types of poly relationships. You've probably heard of these
https://www.discoveringpolyamory.com/polyamorous-relationship-models
"And I hate to say this but often I've seen people not want to be in a monogamous relationship for selfish reason, and threatened to break up with their partner if the other one doesn't accept opening the relationship. "
It's unethical to force someone into a relationship dynamic they don't want. If someone wants to be poly they shouldn't drag a person who wants to be monogamous into it. It's selfish if it's disregarding the needs of others. Someone who wants to be poly should find people to be poly with and not try to hurt others. I want monogamy but in my fatigue I'll settle for something else.
Relationships are complex and I believe that kindergarten comment was from a terrible person trying to take advantage of your good heart. Terrible people seek out good people to try to take advantage of them.
"I think these relational dynamics are complex and most people aren't mature enough and don't know themselves to the point where they are capable of navigating it in a respectful way. I'm not against it, I've seen people do it in a way that was cool.."
Totally true
"i just don't think it's common, and I feel like a lot of people lie to themselves in these situations."
Not sure what you mean. Like what's not common?
"You can also still cheat while non monogamous, so cheating people in monogamy aren't a good argument for it"
I think some people need to not have any commitments and do relationship anarchy. I think some people aren't able to handle relationship commitments.
"I also think it's completely different when couples hire an escort, or when a woman tells her husband or partner that he can see providers because she isn't interested in sex. In a dating context, these situations may cause hurt feelings."
Makes sense.