Alef:
As for the punishment itself, my intuitive reaction is that it might have been better if he had given you a real spanking. That would have given you something else to focus your anger on - as it was, you were forced to concentrate on your humiliation and his betrayal. The single tap changed what might have been regarded a simple, but misguided act of punishment into something that can only be interpreted as a brutal act of enforcing his will - and breaking yours.
Well, I tend to agree with this.
By administering only a single, light 'symbolic' swat, her father seemingly made the exposing of her derriere much more humbling--if he'd administered a sound spanking, then her teenage bottom being bared could perhaps be accepted as a pragmatic matter of making the punishment more effective, but this way it strikes me as having been an attempt to humiliate the victim rather than one to truly discipline her.
It was certainly a strange (and risky) ultimatum to deliver to, and enforce upon, his underage daughter, I'm figuring... --C.K.