I keep it simple.
How would I feel if my partner cheated on me?
Do onto others, as you would have them do onto you.
Ian
Ian ... you got to my reply before I did ... good answer ...
Another good answer - watch the movie "Fatal Attraction"
http://www.imdb.com/title/tt0093010/I started with our band in NY when we were in high school ... As we eventually all got married our wives became close friends as well ... We became such close friends that when it came to extra marital sex, we would be ashamed of letting our friends down as much as as our wives ... we would all 'protect' each other from the advances of other women ... that made it easier ...
But Ian's answer is still the best - how you YOU feel if your wife cheated on YOU ???