I am curious how this case could be affected if prostitution were legal. The act would then be reduced (elevated?) to simply a service being provided. As I see it, it would be much easier to argue the "theft of services" defense if the prostitute is, at least in a legal sense, simply providing a service.
If prostitution were legal and she showed up for her "date" and was told she wouldn't be paid and she was forced to have sex, it would still be rape.