Nice one this (sorry I took a long time to get back to you). Funnily enough, I just taught some material on this last semester, on a course called Leisure and the State. My own views, for what they are worth, go like this...
 
The State reserves the right to regulate and ban harmful sexual behaviour. Of course, it all depends on what counts as 'harmful'. Some people might define harmful to include behaviour that physically injures people,or that results in (costly) medical treatment (which might cover some of this sort of sadomaso stuff?). Less tangible kinds of 'harm' include those associated with laws on porno (that they can deprave and corrupt minors etc), or cause public disorder (assuming they take place in public). Private and consensual behaviour seems to meet most of these objections though? Maybe the argument here might be that such behaviour could escalate into the equivalent of 'addiction' or 'hard' sexuality (such as snuff movies)?
 
Laws might be reflecting more general taboos, of course, rooted in cultures. For some people, sadomaso sex might be 'unnatural', a 'perversion'. The 'natural' kind of sex would only be between consenting heteros where it leads to childbirth? Freud is often invoked here, of course, since he saw 'perversions' as resulting from blocked perceptions of adult sexuality, usually arising from some typical infantile misunderstanding -- maybe the infant saw daddy and mummy having sex and it looked as though daddy was hurting mummy and that she liked it. Or they were caught masturbating by a responsible adult who spanked them soundly and set up this association between the one and the other. Adults develop more 'normal' kinds of sexuality as they realise what was going on, but some people never do and these infantile perceptions remain to dominate (sic) their own sex lives as adults.
 
Sociologists might have a different account of taboos, of course, as 'surface' ways of enforcing some basically functional or socially adaptive behaviour. The usual case here is the taboo against incest, which a number of thinkers have explained as necessary to prevent threats of social confusion -- if you impregnate your sister, is the resulting child your brother or your nephew?  This might be OK but it won't really fit sadomaso which seems a much less socially-threatening form of activity? Others connect sexuality with a desire to regulate and control. Foucault argues (to be very brief) that talk about sex is very useful for would-be regulators since they can spin out a whole discourse involving the rights and duties of adults versus kids, men versus women, population control, control of the body,.regulation of desire and all that stuff. That's why there is so much public interest in sex for Foucault,and so much attention given to it by busybodies and 'moral entrepreneurs' ( to borrow another angle -- 'moral panics' help strengthen particular groups in society, notoriously the police and politicians who gain a lot of attention and resources in their heroic fight against the forces of perversion). Foucault also suggests that sex has to be 'talked up' a bit, to encourage people to do it. We might be able to see some forces opposing the imposition of taboos here? Sex therapists, for example, might encourage a little experimentation to revive jaded married heteros -- perhaps a mild bit of sadomaso included?
 
Other forces against moral condemnation are to be found in the increase in 'lifestyle politics' which a number of theorists of postmodernism and modernity have written about (including Giddens). As the old constraints against non-vanilla sex have weakened (with the demise of the classic family, say), people are now more culturally liberated and are able to explore their own sexuality. This is a big theme in the work of people like Plummer advocating gay rights. Hetero sex is NOT especially privileged as 'natural' says Plummer -- humans can and do sexualise anything (and desexualise anything), since fantasy is so important. Maybe those fantasies include a little sadomaso, chosen deliberately rather than representing the force of some dark infantile compulsion? You could see those who oppose sadomaso as the 'fundamentalists' EITHER having their last blast against an inevitably more 'cultural' form of sexuality OR as the inevitable result of people wanting to get back to some ort of secure reality having been panicked by postmodernism.
 
I dunno if any of this is of any use to you at all. One book which contains some of this stuff on sexuality in general is Brake On Human Sexuality, which has bits of Freud, Foucault, Plummer and several others. It seems to be very well-thumbed in our Library,for some reason. An excellent case study of great interest is the so-called Spanner case of recent years (quite a few websites devoted to it --search under <Spanner>). A group of consenting males got together for a sadomaso sessions involving, for example, nailing each other's penises to planks of wood. They video-d proceedings for their own amusement. No non-consenting people were involved, no minors, no publication of the video was planned, no one needed treatment in hospital afterwards - and yet the police got to hear of the episode and prosecuted the men for assault. They were convicted and punished. Their appeal to the European Court was not upheld -- so lots of good issues to discuss there. I am sure many fetishist sites will also help in explaining their pleasures, trying to calm any fears of real violence, and in arguing for rights for sadomasos.
 
I hope you won't mind, but I think I will include this on my Q and A section ( without naming you, of course). Apart from anything else, words like 'perversions' should drive up the hits most impressively!
 
Good luck with it