As a marriage counselor and sexologist I have been asked this question on more than one occasion. This is not always an easy question to answer, but one that I have answered and continue to answer since it seems to be a topic of discussion. The true problem with this question is that it is entirely too vague and there really is no true moral answer but I have created a list to at least help a man who is undecided on what to do.
These are all yes and no answers
1. Was it a one night stand?
2. Did you wear a condom?
3. Do you have remorse over the situation?
4. Have you gone to the doctor to double check for any diseases or HIV?
5. Are you willing to work on the marriage so that this does not happen again?
6. Your wife does not know this person nor has any association with her?
7. You are positive that you could not have gotten her pregnant?
8. You are positive that she cannot get a hold of you nor has your telephone number or address?
I you answered NO to any of these questions you should let your spouse know what you did, if you answered YES to all of these questions, and were truthful, then I would recommend keeping it to yourself. As I states this is my opinion and my recommendation and the reason that I have put the above questions are for the protection of the wife in every regard, by telling his wife it will hurt her and may end the marriage, but clearly the choice is yours. You are the one who committed the act and it will be your burden to bear!