It matters a lot to get to know your body, not just what you feel good doing and do not like, but probing how much you want, how your muscles feel and how you're affected afterwards. Submitting to a spanking that didn't feel that extreme when it happened, three days later you may feel as if someone had stabbed a muscle (it's the same if you fly off your bike sometimes).

And I guess it's useful to keep in mind the difference between fantasies and the real thing. Some things that are potent fantasies are physically all but impossible (like, being deep-throated through a ring gag - it's too narrow to really let in a heavily erect cock all the way - or being flogged just endlessly while hanging from a hook). Sometimes I have an issue with some kinds of hardcore porn because I feel it pushes off those boundaries completely, but I guess it's also about in what kind of situation those images are seen and used.