This could be the unfaithful spouse's selfish way of relieving his own guilt. He would feel better about his unfaithful actions, and feel better continuing those actions knowing that you were doing the same. Maybe he is getting to a point of actually feeling guilt and remorse for being unfaithful to a faithful spouse and the only way he knows to make himself feel better is to encourage you to do the same (because changing his behavior is probably an overwhelming thought and something he is not yet willing to face).