The modern research picture is considerably less dramatic.
A systematic review of dozens of studies found that BDSM fantasies are far from rare. Depending on the study, definitions and sample, a substantial proportion of both men and women reported such fantasies, while actual participation in BDSM practices was less common.
The more important point is that contemporary evidence provides little support for treating an interest in BDSM itself as evidence of psychopathology.
One well-known study compared more than 900 BDSM practitioners with a control group. The BDSM group did not appear psychologically “worse”; on some measured characteristics, their scores were even more favorable. That certainly does not mean BDSM improves mental health — this kind of study cannot establish such a causal conclusion. More recent research has likewise failed to support the old stereotype that BDSM practitioners are necessarily psychologically dysfunctional.
So a more useful set of questions today is not “How unusual is this?” but:
Is there voluntary consent? Are boundaries respected? Can someone stop the interaction? Is anyone experiencing unwanted harm?
Unusualness by itself is a very poor diagnostic criterion.