Unmarried cohabitants can provide rights to one another that are analogous to rights granted to married couples by entering into a contract or contracts with one another. The validity of such agreements was the subject of the well-publicized case of Marvin v. Marvin in the California Supreme Court. In this case, the court held that an express or implied agreement between a couple living together outside wedlock to share income in consideration of companionship could be legally enforceable. The majority of states now recognizes these agreements, though many require that the agreement be in writing. Only a small number of recent cases have held that contracts between unmarried cohabitants are unenforceable.

When an agreement expressly includes consideration of sexual services provided by one of the parties, a court is more likely to find the contract unenforceable. For example, if one partner agrees to share his or her income in return for the other partner's love and companionship, a court may find that the contract implicates meretricious sexual activity, and may refuse to enforce the contract. Proving an oral agreement or an implied contract between unmarried cohabitants is also difficult, and several courts have refused to recognize such an agreement due to lack of proof.