Interesting thread all around.
The second person, sure. We see it happen here. I think there can be an allowance for confusion if someone is non-monogamist but brought up in monogamy. However... I would hazard a guess that most cheaters end up not being able to stay with the person they initially cheated with.
D has this- wiring. I've known him through at least one monogamous relationship that ended up dissolving, largely due to his cheating, but also because he was unhappy with the relationship and trying to be monogamous. That was a situation with a lot of jealousy on one side, that if I recall drove him... kinda nuts.
Given time in a monogamous relationship, as much as I hate to admit it, I would probably lean the same direction. There's just so many people