Sado-masochism is a private practice that does not harm anyone. As long as these practices occur between consensual adults, no law has been broken. Also, it would be very hard for a lawmaker to define exactly what S and M entails. This is not something the government should be conerned about.
I believe that people should be able to make there own choices, without unnecessary infringement from any government agency. What one chooses to do to ones own body should not be legal fodder for any court. Why should someone who is not being harmed in any way be in charge of passing judgment on someone who is harming no one but themselves? The person harming themselves should seek help not have the behavior attached a criminal label. The stigma would only worsen the mental issue at hand not curb the actual behavior.