Best role? Say what?! Monkey Trouble, son!

Oh yeah, I've heard of that camp.

Yeah I'm sorry but having sex with someone you know is married and not separated, in an open relationship, etc, makes you a piece of shit as well. Not quite as big as the person betraying their spouse, but still a piece of shit.

Have you ever heard of a "conspiracy?" The third party in a marital affair is, by definition, a conspirator participating in a fraud. This isn't "Would you go back in time and kill Hitler when he was ten" complex hypothetical ethics. You are enabling someone in breaking a promise in secret JUST for the sake of getting…

Well, that doesn't really seem like you're the kind of person I'm talking about then. If he lies, that's no reflection on you (although if it was me I would be questioning why it is this keeps happening, if it's happened more than once). That being said, once you found out he was lying, did you ever stay with him? …

In the middle of all this handwringing over your friend's consensual sexual activities,

Disagree wholeheartedly. Not fucking married (or otherwise committed) people is basic social contract stuff. Everyone knows how much being cheated on hurts a person. Anyone with a shred of decency won't make him or herself party to that.