Don't think the results of the Milgram experiment apply except to group scenes. Don't know how common they are, but suspect comparatively rare. Rational choice basis for this: coordination problems, hard to get agreement and cooperation among more people. My understanding is that more typically in kinky sex you a have a bilateral situation in which a sub or would be sub has sought ought a top without being subject to social pressure to do so (the social pressure is all the other way in the general society), and the two negotiate the limits and nature of the activities they intend, the sub explaining her/his limits and the top explaining what he will do and expect. If they reach agreement, they do what they do. Is that correct, Brian? That's what the books say, anyway.
[WS:] The problem with the above reasoning is that what appears on the surface as an interaction between two individuals is in fact enactment of socially defined roles, expectations, scripts, etc. Individual consciousness in an Anglo-Saxon myth. What we have is collectively defined consciousness imprinted in individual's brains and framing their entire thought processes - as Emile Durkheim (or Claude Levi-Strauss) argued. The act is individual, but the script is not - it is collective by its very nature. I do not think an individual separated at birth from society can develop consciousness at all, save the most elemental forms of self-awareness cf. Werner Herzog's film _The Mystery of Kaspar Hauser_ under the much more apt original German title "Jeder fuer Sich und Gott gegen alle" which translates "everyone for himself and god against all." http://en.wikipedia.org/wiki/The_Enigma_of_Kaspar_Hauser.
I think that the most profound implication of Durkheim's idea is that forms of individual though are shaped by the organization of society and therefore are enactments of the existing social relations in one form or another. Just as monotheistic religion is a reflection or "theological sublimation" of the father-child or lord-serf relation found in historical societies, BD is a theatrical reenactment of the relations of domination found in those societies (and perhaps used to enhance one's sexual pleasure.) I conjecture, but I cannot prove it at the moment, that just as egalitarian societies did not develop monotheistic religions, they also did not engage in BD-type of sexual behavior.
I think this principle of individuals enacting socially defined norms and expectations reflecting a particular social organization is almost universally valid and applies not just to religion (or for that matter philosophy) or sex, but to the economy and labor relations as well - cf. Barbara Reskin and Patricia Roos _Job queues, gender queues_ demonstrating that women are not merely "passively" discriminated on the job, but actively participate in that discrimination by bringing in socially defined expectation about "women's' jobs" and roles of women in the workplace. Michael Burawoy (_Manufacturing Consent: Changes in the Labor Process Under Monopoly Capitalism) argues along similar lines for male factory workers.
So the bottom line is that is hard to look at BDSM as merely individual practices or individual consent without ignoring the broader social context and social inequalities in which this consent occurs and conditions individual consciousness. This is of course not to say that BDSM "causes" or even "contributes" to these conditions and inequalities - it merely re-enacts them in the bedroom.
Wojtek