First of all your wife was not your wife when this happened...She was an 18 year old girl...IMO, it was her business...The fact that she told you about this shows that she trusted you with this information and loves you...Let's face it 31 years is a long time in a marriage...I, too, am married....Near 51 lust filled years...Before we were married and when we were engaged or near engaged, I had an affair with my boss who was near 30 years older than I am...He was married, but it happened...Short of intercourse, but it was hot....I think back at that time in my life and being this young woman of 21 years old as foolish, but gaining a knowledge about life that is so important in my growing into the woman who I am....Without it, I may have not sowed my wild oats that showed their face...I know my husband well enough that if I told him this that he would accept it...I have never done this and won't...However, what this affair meant to me and the knowledge in life that it has given me in both knowing women and life, believe me I have no regrets...During the course of our marriage many women have come on to my husband too, but neither of us have strayed...Rethink your thinking....Personally, I think you are so wrong that it isn't even debatable...
Good luck,
Caroline