In modern Western culture monogamy is seen as the best, if not the only, form of loving relationship. Monogamy refers to emotional and sexual exclusivity between two loving partners. On the other hand, consensual non-monogamous relationships (CNMR) are those in which there is an emotional and/or sexual non-exclusivity agreement between those involved. Studies show that there are some individual characteristics that relate to choosing monogamous relationships or CNMR, such as attachment, sociosexuality and personality. CNMR escapes the mononormative culture and people who get involved in these types of relationships are stigmatized. The aim of this dissertation was to investigate the role of the type of relationship in the relationship between Relationship Exclusivity and subjective well-being. Two studies were developed. In the first, we sought to build a scale to measure Relationship Exclusivity with contextualized items. The constructed scale had 11 items, divided into two factors (Monogamy and Fidelity), good indicators of reliability and satisfactory evidences of validity. In the second study, the scale developed in the previous study was used to verify the effect of Relationship Exclusivity on the subjective well-being of individuals in monogamous relationships and CNMR. The results indicated that there is only interaction between the Monogamy factor of Exclusiveness in Relationship with positive affects and life satisfaction (factors of subjective well-being), in the group of people in monogamous relationships. The conclusion discusses the relationship between mononormative culture and the choice for monogamous relationships or CNMR, as well as the impact of this choice on subjective well-being.