I wish people who thought like this were just upfront about wanting non-monogamy rather than sneaking around and causing pain and strife for those around then.
Like, my wife (and partner) practice ethical non-monogamy and have fire years. If one of us wants to stay outside of our thruple, we talk about it and discuss how we feel, and then make a decision everyone is happy with. There are times where something is denied (last one was because of a bad partner she ended up breaking up with a month later, who went full ‘you can’t fire me I quit’ on her), but we all work through it.