For me, the constitutive element of an affair is the secrecy. It is the secrecy that leads to the lying, to the deception, to the duplicity. It is the structure of an affair - not the sexual or emotional behavior or what people actually are doing.
What is the relationship between love and desire? How do they relate, and how do they conflict? ... Therein lies the mystery of eroticism.
I say no to a double standard that men can roam and women must stay put at home. I say no to the fact that men are allowed to claim their sexuality and women just have to pretend that it doesn't matter to them. It's resisting poor relational arrangements. An affair is a way of saying, "No. I'm not playing by the rules." And sometimes betrayal is part of that because you deceive somebody else but you feel like you are, for the first time, being honest with yourself. Sometimes when people have affairs, they feel like they have been lying to themselves for years.