I have never been cheated on, nor cheated myself, or been with somebody who is already in a relationship.
I would not do it myself because if I would have an affair (which in my relationship would be ok) I want it to be fun and entertaining and sneeking around to see each other does not sound like that at all...
What did gave me some thought is the assisting in killing somebody. I agree that that is wrong. But in theory it is no different than assisting somebody to cheating, yet I think you have no responsability there... food for thought.
Maybe because I cannot see cheating as a bad thing, yet killing would be a big NO for me?