First - let me say, you made a mistake, a big one, but your immediate and complete remorse is refreshing. I put this in a different category than an actual (ongoing)affair.
Second - you absolutely must figure out what was behind this behavior - if not, then there is a risk you will repeat it when you hit midlife crisis or some such thing (my H did). Figure it out - were you depressed and looking for the stimulation and excitement, do you have issues with how you see women or with not wanting to grow up and be married, fears about commitment and having a baby - what do you think it was? Because you don't really sound like the kind of guy who just did it because he "could".
Third - normally, if you had kids, I would say don't tell her - if you are remorseful and certain you'll never do it again, why put her through that pain? (And it never really does go away completely). But given that you haven't started a family yet, I respect your feeling that she has a right to know before she makes a decision to start a family with you. I guess this is something you should discuss with a therapist before you make a decision. I mean - if you really feel you have dealt with the underlying issue and are positive it will never happen again - I'd lean against telling her. BUT - if this is a reflection of a weakness on your part, a character issue that may surface in other ways later in life - then she has a right to make her own decision, with the facts.
Think long and hard before you do anything. The one thing I KNOW is wrong is to tell her just to relieve yourself of the burden of guilt.
