"I cheated because she cheated. I couldn't get the image of her being with another guy out of my head."
“I cheated because she cheated. I tried to get over it and couldn’t get the image of her being with another guy out of my mind. I was destroyed. Once I cheated and realized the sex didn’t mean anything to me, I got over her fling.” - Salvatore, Age 48
A cheater knows exactly why they had an affair. A spouse who answers the question, "Why did you do it?" by saying, "I don't know," isn't telling the truth. The truth may be complicated (I wasn’t in love anymore) or shallow (She was hot, how could I say no?) but either way, they know exactly why they did it.