Permitting people to do what they consent to is only logical; the ramifications are nearly non-existent and this decision is in that hands of any given person. So long as the participant(s) in acts related to sado-masochism are consenting adults and are not performing any acts that aren't agreed upon, then there is no reason to regulate or enforce any sort of restrictions on this behavior.
No. It would be extremely difficult to regulate something like sado-masochism. This type of sexual life style has always been kept a guarded secret. I believe this is due to the fact that people would be embarrassed to admit they take part in it. This would not change in the event of legalization, which would make any laws regarding it very difficult to enforce.