(It doesn't matter for the point you're making, but in Germany a contract to supply sexual services is legally enforceable, just like a contract to supply any other kind of service.)
That's interesting, I'm not a lawyer, but the way I understand the German Prostitutionsgesetz, it makes sexual services legitimate part of the financial contract - so both parties may be entitled to payment or compensation in various disputes, however, it still doesn't allow "sale" of future consent, it does not imply that the customer has a claim for the specific performance of some service, or that the provision of the service is enforceable; such consent may still be withdrawn at any time, though it may legally result in a claim for e.g. refund of any prepayment for the service.
