Leaving a 23 years girl will be hurting her or even worse. Divorce might not be avoidable if this info comes out EVEN YOU DO NOT WANT IT or though you can keep the current marriage, then happiness will disappear forever. My opinion is that best of best is that no affairs in a marriage. If affairs do happens unfortunately, then deal with it with the minimum hurting to your current life style. Do not need to feel sorry to the young girl, it was her choice to love a married man, so it is her responsibility to keep it underground. Make this statement to her loudly and clearly. As a man, this is your responsibility to keep the hurting to the minimum to both sides.