Our focus is infidelity as seen by the relational family therapy (RFT) model in the context of the differences between secular post-modern and Catholic sexual morality, since RFT therapists are being trained at the Faculty of Theology, University of Ljubljana. RFT theory and practice are presented, as well as couple and therapeutic relationships. Varieties, causes and consequences of infidelity are defined, followed by the Christian view of infidelity and the RFT healing process. Infidelity is contextualized within different sexual moralities. Contemporary sexual morality defines it as a breach of trust in a relationship; Catholic morality emphasizes yielding to temptation and sinning against the holy matrimony. By interviewing RFT practitioners we investigate our research question: does the Catholic educational context influence the work of therapists with infidelity, and how. Therapists retain a neutral attitude toward the clients; they emphasize an ethical and client-focused stance. Catholic beliefs, if present, are a resource for working with religious clients. We also ask if therapists need to cope with differing sexual moralities when working with infidelity. RFT therapists aren't torn over this, separating their professional role from Christian teaching. Additionally, we analyze the RFT work process with infidelity and find openness to various techniques and approaches, following the "use what works" principle. We also investigate the effect of religiosity of therapists or clients on therapeutic work with infidelity. Religiosity of therapists has a potentially positive influence, since it enables them to relate better to religious clients – even though working with them is more challenging.