The Institutionalization of Same-Sex Marriage examines how contemporary marriage operates and is experienced as an institution. It draws on survey and in-depth interview data collected from 116 individuals in married and unmarried same-sex relationships, who recently gained access to legal marriage in Massachusetts. Bridging two disparate fields of study, I use insights in neo-institutionalism to offer a counterpoint to theoretical ideas in family sociology about the “de-institutionalization” of marriage and demonstrate that same-sex marriage is a case of the continuing institutionalization of marriage. I detail how marriage has become taken-for-granted as an expected and necessary relationship outcome among LGBQ people; how it shapes their relationship choices and behaviors; and how it is understood and experienced as a unique means of obtaining social legitimacy. The findings in this study also illuminate the institutional mechanisms through which marriage shapes social action today. They reveal that although the formal rules and informal norms of marriage have weakened, widely shared cultural scripts that connect marriage to love and commitment preserve its institutional force. In addition, this study provides a detailed, complex picture of the consequences of legal marriage for LGBQ people. It finds that marriage can provide them with more defined templates for their relationships, fulfill their desires for commitment and security, and make them feel they belong in their communities and society at large. At the same time, it can also weaken relationships that do not follow the marital path, depress debate and critical perspectives, create new inequalities, and highlight the limits of family and societal acceptance. This study offers family scholars a more comprehensive and nuanced examination of contemporary marriage, institutional scholars a new empirical case study for thinking through institutional processes and outcomes, and sexualities scholars much needed empirical data on same-sex relationships in a rapidly changing legal and social landscape.