I don't think people are suited for open relationships. Some people can do it, but I think it is too much trouble for most people.
Some animals have evolved to be monogamous as a survival strategy. Humans have evolved to be monogamous too, that's nature.
I don't know how those who can do it do it. I once knew a girl who had a girlfriend and a boyfriend. Her family knew them both, but separately. She never went to a meeting with the two of them together. That must be weird for everyone 😅.