The purpose of this article is to show the affective side of the marital infidelity as it could be traced through therapeutic process in relational marital therapy. Relational family therapy is based on the notion of mutuality of affect and transgenerational transmission of affect dysregulation. In the article we present a couple therapy in which we address the affective mutuality of the dynamic of infidelity and uncover the deep shame and fear that lies at the core of that dynamic. In the therapeutic process of the couple therapy a certain turning point in the emotional flow of the process, namely the onset of sadness, occurred through recalling of painful childhood memory of physical abuse and neglect. We argue that the onset of deep sadness created a safe enough environment for the couple in which they could start mourning detached relationships in their families of origin and thus prepared the ground for the verbalization of the affects of fear and shame.