Demented Teddy said:
Anyway about the pins, I'm sure that's only BDSM fantasy with the exception of very rare cases
*ahem*
The pins was one of the milder examples that came to mind from what I might say "history" if I was at a confession, which I am not, so I will just say that trust me, these things do go on.
From a purely psychological perspective:-
It is a lot more common than you might think. The thing about it is that it tends to be addictive and it tends to feed itself. Once you get to a certain stage with it you become extremely desensitized to actual pain and/or psychological torture
during the act. You know when you see stuff on various sites and wonder "how the hell can that person tolerate that?!", it is because your brain enters an almost tranced/narcotized state when you enter the 'zone' as it were, and the brutality actually feels different.
A hopelessly inadequate analogy would be comparing the pain of a bad tooth and the pain you feel after a heavy exercise session. Due to the chemicals flowing through your blood and brain, one sense of pain becomes something you can end up craving, while the other is something you'll do anything to avoid.