If you have an affair, and you get caught, why should you tell your partner all about it? Why "Tell All"? Who among us has a partner that will feel better knowing how many times we got oral sex from our mistress?
Thank goodness I have never been in the position of needing to.
Personally, I believe that shielding someone you Love by omitting some of your "deeds" (which could easily include infidelity) is on the road to "greyness". By greyness, I mean the area between black and white; personally, I hate the grey - because it has no boundaries, and is therefore tantamount to having, as King Henry 8th did - an "Elastic conscience"