Sex and Violence are intricately tied., Thus: BDSM
There is actually a neurological link also.
The same adrenal response happens in both and physiologically your body is behaving the same. In the fight or flight scenario your body is tensing preparing for a fight, your heart rate increases due to adrenaline, thus increasing your respiratory rate, non-essential functions such as digestion stop functioning and this is exactly the same pattern that takes place during sex.
They are both hormone driven, reflexive behaviors.