AbstractSome progressive U.S. Cities and several Canadian provinces now provide mechanisms for polyamorous families to register as such with state authorities. More than a million people in the United States identify as polyamorous and many more practice some form of ethical nonmonogamy. This article suggests that the growing recognition of polyamory poses a substantial threat to a simultaneous development in family law: the call by scholars and the Uniform Law Commission for courts to enforce a more implied contract, implied partnership and equitable claims in the context of non‐marital conjugal cohabitation. Non‐marital cohabitants argue that courts can infer marital‐type commitments to share property from the fact of conjugal cohabitation. They argue that their nonmarriage should entitle them to the kinds of relief afforded to divorcing couples. But polyamorous conjugal cohabitation involves very different norms and commitments to reliance, partnership and transparency than does traditional marriage. Marital‐type relief maps awkwardly, if at all, onto the reality of most polyamorous relationships. By bringing into relief that which we cannot necessarily assume about conjugal cohabitation, the recognition of polyamory questions what many proponents of more legal protection of nonmarital couples ask courts to assume about conjugal cohabitation, namely that it gives rise to reasonable reliance on a status quo and an intent to share property. Moreover, by providing a means for polyamorous households to register their relationships, polyamorous registration normalizes the idea of non‐marital relationship registration. The more normal and expected it is for people who want rights as some form of family to register their familial intent with the state, the harder it is for those who have not so registered to argue that the state must treat them as some sort of family.