I agree with you in the most part. Specifically I agree that consensual S/M (or other kinkiness) between adults is not necessarily pathological. You might walk a into a grey area if you judge it to be pathological for that specific client when the client will not or cannot acknowledge it, but that's different from categorically regarding it as pathology no matter what.
I will take issue with the use of legality as part of your determinant, though. Lots of things far milder than S/M are still illegal in many jurisdictions (consensual sodomy being the most famous example). Should we regard homosexuals living in those areas as sick because they broke the local law? Obviously not, and neither would they regard themselves that way. Their attitude, and probably yours, would be that the law is wrongheaded and intrusive. Unfortunately, this entitles our necrophiliac to attempt the same justification, because if we decide to ignore the law for one sexual act we have to at least consider extending the same allowance to all of them.
So it's a little more complicated than just asking if it's illegal; you wind up judging the justice of the law, which is bound to be subjective.
Back to the original question, though. You did say that the particular example (consensual S/M) "offends you plenty." Given that, how sure are you that you can keep your repulsion from influencing you? Could you bring yourself to unreservedly perform marriage counseling that was intended to bring joy back to whippings?
We like to think we're objective, but we're as human as anyone else, and the stronger the repulsion the more difficult it is to suppress. I'm not sure if I could keep such things from affecting how I handled such a case, but damned if I know where I'd refer it. :)