There is a book MWD quotes in her "Healing from Infidelity" book. It's called "Not Just Friends" and details how this happens and what his thought process is. Remember, while we all have to own what we contribute to a marriage, an A is not about you. It's about him. And the fantasy of who he is when he is with someone that doesn't know what he's like to live with.
Following the advice in MWD's books and these threads can help so much. Focus on yourself. Detach. It will come to your aid no matter what happens down the road.
Thank you, oceangrl. I'm going to check out that book. I'm finding self help books to be a good crutch right now. I'm hoping my lack of reaction is a sign that I am detaching properly.