Ann Launders always put it somewhat this way: Imagine the woman already knows but is trying to keep a little dignity. Then you go and say something. Now she no longer has a faithful husband OR privacy and dignity.
I'm not sure I agree with that, but it is something to think about. If my wife was busy on the side, I would probably want to know about it. However, I would rather a note with enough details to prove the case and a closing that said, "I have told no one and will not tell anyone" Note should be unsigned. We never know what goes on behind closed doors. I have know folks that were serial cheats and I have known women who turned to someone else simply because their husband hadn't gone near them in a while. I see and hear a lot of things and have decided it's all really complicated.