Reflecting on polymorphous perversity and the entire body as an erogenous zone leads to a consideration of kink. Kink in my understanding is roughly equivalent to what has been referred to as "paraphilia" amongst psychiatrists and includes a wide range of erotic interests that are beyond the usual genital couplings of a man and a woman. At one point homosexuality was considered a paraphilia, but no longer is; it seems the new volume of the Diagnostic and Statistical Manual (the major reference manual of North American psychiatry) is removing most of what is considered kink from that classification as well. That change is due to the fact that, as with sexual orientation, there seems to be no widely accepted evidence that those drawn to kink in its many manifestations are any more likely to suffer from psychological aberrations than is the general population.
How we come to have the particular erotic attractions which compose our sexuality is one of the least understood areas of sexual research. There is some indication that those erotically drawn to corporal punishment, such as spanking or whipping, were more likely to experience such punishment as children; however, that doesn't seem to have been the experience of the majority of those so attracted. That is the only exception in research of which I am aware that indicates any differences in childhood experience between those attracted to kink and those who are not. In fact, the only difference I've seen reported in several studies between those drawn to kink and the rest of the population is that the former tend to be more intelligent than the latter.
Among the more widely practiced erotic interests associated with kink are sadomasochism and fetishism (including erotic arousal toward specific body parts and specific materials). Sadomasochism (S/M) involves a power exchange through which one individual dominates another, though that exchange may include a group activity in which more than one person plays the dominant or submissive roles. In a one on one exchange the dominating person is usually referred to as the dom or master and the dominated individual as the slave or sub; the power exchange may focus on one or more elements: control, infliction of pain or discomfort and humiliation being the predominant ones. For myself, most other health professionals and the vast majority of those who practice S/M a power exchange in the context of erotic play must include two conditions: it must be consensual and any harm caused to the sub must not be lasting or permanently debilitating.
The same actions present in an S/M session, when practiced without consent could be considered as assault. Even with the presence of consent, actions that result in permanent harm or maiming to the sub would call into question the psychological health of both participants and in the opinion of many would justify legal interdiction. The simple exercise of control and verbal or physical humiliation in the context of a consensual interaction are not likely in themselves to cause harm. Fetishism, as well, is not in itself likely to cause harm; being turned on by substances such as rubber, leather, PVC or silk or by specific body parts, such as feet, does harm to no one and may bring considerable pleasure. The exception occurs in situations where the fetishist himself feels shame regarding his fetish or finds it limiting or restrictive in relation to the enjoyment of his sexual life as a whole.