My wife slept with someone else with my approval. now she thinks about him when they are apart. Now what? help?

This was supposed to be something to add spice to our relationship. I carefully chose the participant and we have since had a with him. It was fun. However my wife told me that she sometimes just feel that she needs to see him or talk to him outside of our sessions. She has professed her love to me and committed to our relationship but I think that she maybe "falling" for him. i'm not sure if I should end the marriage ...end their "fun" or just let things play out and see if it is just a phase. I love her alot...and this wasn't her idea in the first place...i ma the one at blame if there is any. What do i do?