From what I understand, it's common S and M practice to bite hard enough to draw blood, cut, whip and pierce...most of which leave scars, so in a way it's kind of permanent.  I will concede that S and M is usually for sexual gratification, and the couple that bit off each other's fingers did so as a mark of committment rather than for sexual pleasure.

Bothe are deviant behaviour - deviant meaning ' behavior that is a recognized violation of cultural norms'.  So, unless we live in a society that looks upon cutting, biting, whipping and piercing for sexual pleasure as 'normal' then yes, they are both deviant behaviours.

Froggy!  How are you?

I've perused BME a few time before, so I'm familiar with it's content.  I've read stories of people self-modifying, but I've never come across this particular..ummm....ritual before.  I simple cannot imagine biting off someone elses finger...