This research aimed to investigate the experience of men and women in marriages conceived as monogamous, in the sense of sexual exclusivity. A qualitative research was carried out, in which five men and five women were interviewed, married or in cohabitation, in a hetero-affective relationship and without children. To analyze the results, the content analysis method was used in its categorical-thematic aspect. Based on the analysis of the data collected, five main categories and one subcategory emerged. They are: Pillars of conjugality; Sex in marriage; Sexual exclusivity in conjugality; Marital infidelity: when exclusivity is broken; and Conceptions about non monogamous consensual relationships. The category Sexual exclusivity in conjugality was divided into the subcategory Dissociation between sex and affections. The results indicate that the sexual exclusivity agreement is related to the search for emotional security and marital stability, with affective exclusivity being more valued than sexual exclusivity. Dissatisfaction in marriage and the desire to fulfill sexual fantasies were identified as the main predictors of marital infidelity, which was defined as a breach of what was agreed between the members of the couple. Non-monogamous relationships, which do not require exclusivity in a consensual way, are unfamiliar and considered complex by the interviewees. It is concluded that the sexual exclusivity agreement is implicitly assumed and rarely discussed by the spouses.