This Article addresses the regulation of the relationships between unmarried cohabitants. It challenges the conventional divide between conservative and liberal approaches. On one hand, moral condemnation of nonmarital conjugal relationships and public policy in favor of marriage lead conservatives to reject the application of marriage Law to cohabitating partners. On the other hand, based on principles such as freedom, tolerance, and equality, liberals tend to equate the mutual legal commitments of cohabitants with those ofmarriedpartners. I break with conventional analysis by offering a novel liberal model that separates between the mutual obligations of cohabitants and married partners. The proposed model is based on a pluralist constitutional theory that underscores the responsibility of the liberal state to create a range of social institutions that offer meaningful choices to individuals. The Article thus argues that the Law should develop two distinctive legal regimes for marriage and cohabitation and provide couples with substantive freedom to choose between them. It offers arguments based in morality and efficiency to support the proposed bipolar default systems and the role of marriage as a screening mechanism. The Article further shows that this pluralist theory goes well beyond standard cohabitation Law and provides a frameworkfor an innovative pluralist regulation of spousal relationships. It demonstrates the potential contribution of such a framework regarding three publicly disputed topics: same-sex marriage and civil unions; covenant marriage; and secular regulation of religious marriage.