A number of couples request sex therapy because of relational problems resulting from an extramarital affair. In treating such couples, five phases have been identified: (1) the start, (2) suspicion and negation, (3) explosion and impulsive reactions, (4) making choices and deadlock, (5) working through. Clients may enter therapy at very different points in these phases. Some come in full crisis, others in the deadlock phase, not being able to decide what to do. A different therapeutic intervention is proposed for each phase.