Well, I'd have to sit down and evaluate the situation...there are many variables that need to be considered.
First and foremost, in some issues, cheating results because of emotional abandonment by the spouse who has been cheated on. While this doesn't justify the behavior, there needs to be a serious evaluation of the circumstances involved. I do tend to agree with CS, Brad, and Poetmom that the presence or absence of children may make a difference in how you react, but I have personally never been a person who has given my love and trust easily, and I would have to consider a lot of things before I could give an adequate answer.
As for my wife (we have been married 9 years...together 11 years), I have invested a lot in our relationship, as has she. If she were to cheat on me (or attempt to), I would seriously evaluate what part I needed to play in repairing our relationship (there was a time when I was working 70-80 hours a week to support the family; I had to stop that and evaluate what was really important to us; if my wife had cheated on me during that time, I would have to consider my own behavior a factor in influencing hers).
I realize this is the biggest non-answer given since Clinton left office, but...I think the question is too simple for a simple answer.