I have a few friends who have cheated on their mates due to alienation of affections or having lost a physical desire to be with that person for whatever reason. They knew at the time that they were wrong by having a physical and emotional affair. In the end they regretted it and the irreparable damage it did to their relationships.
It's not always this cut and dried. People cheat for many other reasons and there are many other outcomes than irreparable damage to the relationship. In your friends' case, I might venture that the end was coming anyway and the affair was just a death knell. Who knows. I just know that cheating is just not that simple.
If you are the person who has been cheated on there is nothing anyone can say that will justify that persons actions. If you are the person who went outside of the relationship for sex you will most likely never believe you were wrong in your actions.
Holy blanket generalizations, Batman!
As an aside, if I read "Doubt Means Don't" one more time, I'm going to hunt down Oprah and skull-fuck her till she dies.