you're a caveman.  The cavemen really knew how to slap their women around.  Usually, modern kink is distinguished by sensual overload and a slow build up to a major O.   Women that want to be a punching bag have usually never experienced classic sexual domination, and therefore have a distorted idea of what its all about. Next time you give her a good beating, check and see if her pussy's wet. There's your answer on whether she really enjoyed it or not.  

 
I have known a few girls that claim to be able to O from being paddled on their bare ass, but I have met many more that will respond better with a much more explosive O to restraints, a blindfold, and a light, teasing touch to various erogenous zones with everything from fingers, to metal objects, to ice cubes.  When they let you put cuffs or other restraints on them, or insert objects into their pussy that they can't see coming because they are blindfolded, then you know you have a real submissive.  However, they have to be comfortable that you know what you're doing and trust you completely not to injure them.  Spanking and hair -pulling to not build trust.  In fact they often achieve the opposite.  You have to ask yourself if the abuse you think she likes isn't just her way of punishing herself for selling sex.