Subcultures sit within and define themselves in opposition to the superstructure in which they are located. We operate in a universe that functions via difference, both linguistically and in terms of human interaction. I am not you: my identity is partially determined by its not-you-ness, the same is true for groups. Both the mainstream and fetish societies are reliant upon each other for their continuing existence and meaning. We are not them, they are not us.
This is particularly interesting with reference to BDSM in that a lot of what we do is based around transgressive acts, that which is forbidden or dangerous is appealing. The societal antipathy towards degredation, for example, gives it some of its appeal, similarly pain, punishment and a host of other practices are determined in part by their difference to what is acceptable behaviour in the mainstream. Images of transgressive behaviour are stimulating because of that air of forbidden fruit.
The boundaries are not fixed. Mass media, in particular, makes use of representations of BDSM culture to create a shock effect or an enhanced, eye-catching eroticism that by its increased use ceases to be either shocking or eye-catching. Over time, that which is outsider, strange or marginalised can become absorbed into the whole, legitimised. Teenage rebels become tomorrow's advertising executives and bring a little of that rebellion with them to make the sale, but no so much as to put people off.
Following this process through, could there come a time when BDSM was absorbed in such a way, when a collared slave and Dom(me) walking in the street looks like a couple holding hands, and what effect would this have on how our desire operates? The key element here is that when what we want is powered in part by it being forbidden we need to move the boundaries once our previously transgressive act is legitimised, what was once exciting can become tame by comparison.
The theory of evolution posists that a change in the genome occuring through sudden mutations can produce a quantifiable difference in a species which has a knock-on effect on its ability to survive and reproduce. It is not a gradual process, but instead a leap, which comes into direct challenge with the norm. The two groups, each with their different code, compete for the limited resources of their shared ecosystem in an ongoing struggle.
Societies and political movements can operate in a similar way, except that the ecosystem is linguistically and ideologically constructed. I suspect that such jump in attitudes is far off, although we can look at gay and lesbian subcultures for a model on how shifting perceptions can alter the way a society operates, and certainly this process is nowhere near complete. For the moment we are safe in our subversion.