I ask myself this all the time.
I've been involved in my local BDSM community for about 5 years, and the question comes up now and then (usually with the attitude of "how could they possibly think there's something wrong with me"). Honestly, when I hear hostility towards the mere mention of the thought, I think people are afraid to analyze something that really, should be analyzed.
* A prior history of abuse, especially among females, is rampant in the BDSM community. Perhaps as one poster suggested, its just as common in the vanilla world. My gut tells me that the rate is truely much higher than society at large.
* A large segment of society thinks our behaviors are "sick". Of course, the community argues (and I agree) that our lifestyle is merely misunderstood and we're really the upstanding members of society that live next door to them (but then again, that's also true of child molesters). Isn't the "misunderstood" lines what child molesters, thieves, Enron execs, and other malevolent members of society claim, too? Are we deluding ourselves like the heroin addicts who can "stop at any time"?
* Most of the behaviors that we fantasize and act out would be serious felonies in the real world. Others point out that everyday people fantasize about horrific acts too. I'll admit, especially when I see some 'foolproof' plan, my first thought is how I might defraud/defeat it. But the difference is, the defrauding remains a whimsical musing. In contrast, I actively seek out partners to perform BDSM on.
At the end of the day, I'm troubled by the fact that the crazy person is not the best judge of whether they are crazy or not.
In considering the above, there's two things that let me sleep at night:
1.) She likes it too, and we're happy. Its been my experience that BDSM relationships are deeper and more fulfilling than vanilla ones.
2.) I don't know anyone personally who's left the lifestyle and has "been cured" and came back and warned the community what they did was wrong. That can't be said of a lot of the groups that use the "misunderstood" line.