In this study, we examine whether there is a well-being gap between persons in same-sex and mixed-sex unions. We consider the possible role that tolerance of homosexuality plays in the size of this gap by comparing these union types across nine European countries with varying levels of normative and legal tolerance (informal and formal institutional contexts, respectively). For social well-being, results indicate that the well-being gap indeed depends on both the informal and formal institutional contexts in a country. In intolerant societies, persons in same-sex unions have lower social well-being than persons in mixed-sex unions, whereas they fare slightly better in tolerant societies. We found that the normative dimension of tolerance matters more for the social well-being gap than the legal dimension. In line with our hypotheses, findings also show that men in same-sex couples are more affected by differences in the informal institutional context than women in same-sex couples. For depressive feelings, no significant joint effect of union type for either measure of tolerance was found. Overall, we demonstrate the theoretical usefulness of treating tolerance at the contextual level as a predictor of well-being by empirically testing the link between tolerance and well-being. We suggest considering intolerance also as a social problem on the country level, which is distinct from intolerance on the interpersonal level.