This is where things get tricky.
As I stated earlier when cryxis pointed out that his particular sect of christianity did not perform physical acts of self-injury as part of a religious devotion, many forms of protestant practice outright reject the physical body-- precisely because of statements like that one in Gallatians. (However, such rejection takes on a more abstract form, in that the rejection is not taken so... Litterally... The feelings of the body are of the body, so punishing the body does not actually reject it. Refusing to act on the impulses or sensations of the body, such as abstaining from lusts, or from over eating, etc.. is more in line with the practice done by protestants.)
Ironically, the self-flagellation performed by the flagellants in the 13th and 14th centuries derives from the same source. It has its roots in the literal application of the metaphysical pronouncement to "Die to the flesh".
http://en.wikipedia.org/wiki/Mortification_of_the_fleshIn this case, they are "rejecting" the flesh, physically, by performing devotional self-inflicted injury. This process causes endorphins to be released into the brain-- It is a natural part of feeling pain. The flagellants did not KNOW this, however. They instead noticed euphoria associated with their recitation of their prayers and devotions while simultaneously denying their flesh, which brought about a very profound "Spiritual" experience.
In their minds, they very really were being brought closer to their god by this act, and they considered it very much a spiritual thing.
This does not change the fact that experiencing pain releases endorphins, and that one can become addicted to these endorphins, or that these endorphins can greatly increase feelings of euphoria, which is exactly why BDS&M is able to function as a sexual fetish.
It is further colored by sex itself having frequently been employed in religious rights of non-christian origin, such as the notorious vestal virgins of greece and rome, and also described in ancient mesopotamian texts. (One such sexual priestess is mentioned prominently in the epic of gilgamesh, for instance.)
Christianity rejects such practices as base and abhorrent, but the practitioners of those religions believed that such sexual acts were very much sacred and spiritual in nature.
The pace de resistance on top of the thing which really sets the "Funny" (as regards to BDS&M), is the costume worn by the 13th and 14th century flagellant:
(And now that I have thoroughly ruined the joke by completely explaining it, and removing all joy from other readers by preventing them from finding it themselves after digging)
The flagellants of the 13th and 14th century were human beings, just like everyone else. Much like the recent scandals that have rocked our own century with pedophile priests in the clergy, there were "deviants" back then as well, and the flagellants were no exception. I do not doubt that some of them reached levels of ecstacy suficient to well--- you know.