That's an interesting question. I'm pretty sure I knew about BDSM in general before I realized I was into it, so I never really thought of it as abnormal in the first place. (Since I've known I'm into BDSM since I was 10 years old or so, you can probably chalk this up to my liberal San Francisco upbringing. ^_- )
I think this is precisely what psychologists are trying to get at with the clinical definitions of paraphilias. A paraphiliac disorder isn't just having a kink, it's having a kink and not being able to deal with that in a healthy way.Not that I'm suggesting we are abnormal but ... Well, I guess I'm just thinking Freud and the concept that, once you cut through all the crap, it was not their desires that made his patients ill, but their repression/denial of those desires.
Take eating disorders. Nothing abnormal about eating. And yet there are people out there who just cannot come to terms with their need to eat. Probably a bad example, but hopefully you get the point.