Joan decides that she wants to have a child and become a single mother. She therefore decides to ask her friend, Preston, to act as sperm donor. Preston is college-educated and has a good job - he agrees to be a sperm donor as a favor to a friend. The two sign a contract in which Preston agrees to give up all parental rights. After the child, Javier, is born, Preston changes his mind, suing Joan for joint custody based on the idea that the contract is not enforceable. What argument is he likely to make in support of this?