This study analyses how the adoption of legislation in favor of same-sex unions in the US states transformed the institution of marriage by including gender equality values and moving it away from a traditional view mainly articulated on the basis of religious beliefs. While the literature on morality policy allows identifying the scope conditions within which such institutional change has happened, the reasons for which some state governments legislated in favor of same-sex marriage amidst federal and state efforts pushing to preserve heterosexual marriage remain largely unexplored. By combining the theoretical frameworks of rational-choice and historical institutionalisms, it is possible to delineate a set of conditions under which actors brought about institutional change. Qualitative Comparative Analysis, a medium-N technique, further allows filling the gaps in the literature by shedding a new light on configurations of conditions and without leaving aside exceptional cases. The findings identify several causal paths that led to institutional change and underline the importance of taking into account conjunctural causation and equifinality. In some states, legislation in favor of same-sex marriage has been added on top of already existing legislation allowing for civil unions. In other cases, marriage equality has directly displaced the traditional marriage institution as the result of LGBT advocacy groups’ efforts. In both instances of institutional change, protections clauses for actors who opposed same-sex unions on religious grounds were a condition sine qua non for the legislation to be adopted, thus pointing to the strong influence of the Catholic Church as an interest group.
