Men you have to obey the boundaries of the game in which you are playing. Think about it like this: in soccer when someone operates outside the established set of rules within the game, they are shown a red card and dismissed if the offense is great enough. If not, perhaps they receive a warning (yellow). Two yellows and they are dismissed.
Same can be applied to relationships. Establish the boundaries of your relationship early, and when someone within the relationship operates outside of those boundaries, they must be dismissed. Boundaries and rules are so important because they end up making the decision for you. No matter how hot, how great of a woman she is, if she oversteps her bounds she is given a red card and dismissed.
For me cheating is a red card and she should be dismissed immediately, never to be spoken to again.