The Sugars' exploration of infidelity continues with a conversation with Esther Perel, a relationship therapist with extensive insight and expertise on the subject. What qualifies as infidelity? Why do we go through with it? And, perhaps most importantly, what does it mean to us? If you haven't already, listen to Parts 1 & 2 of the series, in which the Sugars heard from the betrayed and the betrayers.