In this article, we use comparative historical analysis to explain agenda-setting and the timing of policy outcomes on same-sex marriage in the United States, Canada, and Australia. Unlike the United States and Canada, Australia does not have a bill of rights, making litigation to obtain rights not enumerated in existing legislation unavailable to activists. Extending the literatures on the development of public policy and on political and historical institutionalism, we argue that in the absence of domestic opportunities for legal change, international law becomes more important to activists in wealthy democracies, but it is contingent on states’ specific institutional and cultural features. Even when international law is "domesticated" into national political structures, it is still secondary to internal conditions in countries with extensive rights-based polities. International law may set a political agenda, but once introduced, policies move according to internal conditions related to party discipline, the centralization of courts, and policy legacies within those countries.