That can is rendered entirely meaningless by the conditions in that statement. In the same sense you can also declare yourself king of the USA.
Becoming a slave can essentially physically make you a real slave and both parties of the contract can live the rest of their lives as a slave and a slave owner. It's only the ephemeral concept of law that doesn't happen.
They're essentially completely opposite scenarios.
For a closer example to the original consideration, assume a person that is abused by their spouse: There is a good reason why the abuse will be prosecuted regardless of that person's wishes if authorities are made aware of the abuse.