I think a lot of wrong conclusions have come out of studies that have focused only on offenders, because there is more incentive to study offenders, and they are more accessible.

I disagree. I do not disagree that there is more incentive, we as a society need to understand in order to recognize and to prevent/protect. Therefore, I strongly believe that the conclusions that are drawn are not in any way wrong. I also do not disagree that they are more accessable.
I'm trying to do this from memory, I believe that for anything to become abnormal is a continued obsession that lasts longer than 6 months and is the only way that one can find sexual release. And obviously it is anything that is beyond a vanilla heterosexual relationship. For a long time homosexual relationships were considered abnormal. And that changed. The problems begin when these "obsessions" are acted on. There's the line. This is inherently important because when looking at a pedaphile....they can't be treated. The only thing that is going to happen is that its going to escalate. (sp?)And that goes for any of your sexual predators. Thats good to know.
It is recognized that these relationships (BDSM) are not harmful providing they are consensual. That is recognized. Its still a paraphiliac.
And as a side note that is completely not related: there is a huge fear that if we as a society say that homosexuality is ok, whats to stop us from saying pedophiles are ok.......they are on the same list. See?
I don't think its a question of it being common, because whether we say its common in the general public doesn't make a difference. I mean, its 2004, I would be hard pressed to find a large population of people that didn't come from a dysfunctional abusive back ground.