People who are into BSDM stuff do so with the explicit understanding between those involved that everything that happens must be on an entirely consensual basis. This consent may be revoked at any time if unwanted discomfort, whether mental or physical, intrudes upon the proceedings.
Further, by having the right to grant and take away consent, the submissive party is actually more 'in control' than the dominant.
Having recently taken in a rather energetic stray cat, I ultimately see it as being not very different from playing with the use of catnip toys and such. Though their inherent cuteness makes it easy to forget the fact, cats in reality are vicious predators who have a strong instinct to tear apart and eat small animals. Playing with them allows them to channel their inherent aggression into a non-destructive form, and helps to prevent their unfulfilled desires from turning into what we would deem to be 'bad behaviors', such as scratching and biting of humans.
If you'll follow the analogy, human sexual desire is really not at all different. The behavioral problems that result from prolonged repression of instinct may ultimately turn into real violence towards women if allowed to go unchecked. People who are into BDSM are frequently misunderstood due to the outwardly aggressive nature in which they express themselves, but if approached from the proper perspective of consent, their form of 'play' is ultimately more healthy than that of many who would presume to judge them.