I know this is not the outcome you wanted but IMO it's the right move. You have to be true to your values. People who have physical affairs rarely change and typically only change when they hit rock bottom. The early years of marriage are suppose to be great times as your build your life together. If she is stepping out now she either feels she made the wrong choice or she has deep rooted issues.
Learn from this experience and move forward. You absolutely have a great life ahead of you.
