Her naked skin

You look up at her. "Right now? I actually feel stressed because of your hands touching me while my wife does not know I'm here. I can feel your naked leg against my arm and wonder if it is right what we do here. I'm turned on by this massage and I should not be." Then you stop. It's out. Now you are afraid how she will react.

Her hand is still massaging your neck. The music is still playing. The scent of candles is still the same. Yet, there is now a tension in the air.