Arousal from receiving
flagellation.
This is a form of phobophilia common among those who participate in S&M sex.
Apparently, this was popular after the Black Death in the Middle Ages. People felt the need to exercise control over themselves and their fates, and so two sets of extremists arose. These were those who participated in orgies, and those who participated in self-flagellation (the flagellants). The flagellants originally whipped themselves in order to punish themselves for the sins of the clergy and society. In a stunning twist, however, some later orgies consisted of whipping combined with sex. I'm sure that would've been a lot of fun.
I believe my history teacher said it best:
"Whipping always leads to something, whipping just doesn't stop. Whipping. Stop. No!"