That's the most important thing about monogamy is it shows who you really are. When you step outside of your MR, be assured you have shown that you can not be trusted. Too far? Not really. Here in Las Vegas, if you have been fired over a substancial money issue or have been convicted of a monetary crime you can expect to not work around money again. Sure, it was only one bad decision made in the heat of the moment, but you still stole the money and it will follow you forever. In a MR, if the other person ever trusts you again after cheating, it's because they are a saint. It shows you don't respect the boundaries of your relationship. Too far? Nope! If you work for a large corporation and are seen cavorting with a rival in a way that might compromise company information bestowed upon you, you'll probably get fired. Cheating shows that you can willingly hurt the person closest to you without shame. Chances are, if you had shame, you would have thought about your partner before thinking about yourself.
It's not wrong in my book to sleep with multiple people. The wrong happens when you refuse to be honest with all the people you are sleeping with. It's wrong to lead someone in a relationship that they think is "special" and then to go off and do your own thing because you think you won't be found out. There are plenty of people in the world who want to have free, fun, casual sex. They're usually open about it and willing to admit that they don't want to be tied down.
Hopefully, in the future we will be able to be more honest with each other, that way we can build lasting relationships that actually mean what we say they mean.