My theory on the "should you tell him" thing is this:
Being unfaithful is one thing, but telling him about it would be cruel.
Telling him about it will just send all kinds of doubts into his mind. ie. Was I a good enough lover? Can I make her happy? Can I trust her again? Why was I not enough for her?
This uncertainty will last for years and years and years--whether the two of you are still together or not.