Interesting question.

If you were the sort who got in lots of fights, eventually being punched might become erotic as you knew what to expect, and were therefore less afriad of it. I knew I guy who started punching his mate during sex because slapping her woke up their baby. She grew to like it. Or so it seemed.

I believe, partly based on my own experiences, there is a fine line and a lot of it has to do with mood, the set up, so to speak. What was the conversation before the brutality?

If you were were whipped with a ruler in school as punishment how could this become an erotic fixation? If you were a spanked little girl, why continue a family tradition? You likely hated it as a kid, but the effect lessened everytime...and became...fun???

For some being tattooed is erotic pain, they come out bloody and ready for action, others not so, it is simply a process of art. They might like the endorphen rush, yet it is non erotic. I think it is all mindset.

Think about it, what happens when you feel pain? It changes your body chemisty, similar to a drug, and if it is something that actually scares you (not the, oh no, please don't hehe kind of scared, the I don't want to be punched in the face and lose my teeth kind of scared) this would send different chemicals flooding your nervous system.

I believe, psychologically, almost any pain could become erotic. Cutting limes with papercuts on my fingers hurts, but if there is a damn sexy guy flirting with me at the bar as I do it, it becomes a secret erotic moment.

I am sure there are studies on this in progress as we speak.