One of the core principles of social democracy is the equality between citizens. However, in Europe, several social democratic parties have recently supported referenda against same-sex marriage. This paper aims to examine factors which drove left-wing parties to disregard the principle of equality by comparing the four most recent same-sex marriage referenda in Croatia, Romania, Slovakia, Slovenia between 2013 and 2018. Results of the qualitative analysis indicates that these behaviours can be explained by a combination of electoral decisions, government strategies and other ideological components.