Wife Has Run Off With A 20-year-old, What Do I Tell Our Son?

The affair started a month ago. As soon as I had proof (disgusting text messages with "I love you", heart emojis, and sexual things) I confronted her. She didn't argue, and certainly didn't fight for us. She just left to be with the other man.

He is 20. Half her age! Only 4 years older than our son!

Our son keeps asking when mom is coming home. Eventually I am going to have to tell him that his mom has abandoned us to be with a young man.

How can a woman abandon her own child, and her loving husband of almost 20 years? What does it teach our son? That women can't be trusted?