You seem to be saying that it's fine as long as it's consensual and no one is being harmed, and I agree with you on that. This is one of the reasons I find DSM-IV sort of lacking on this particular point (and the reason I chose this particular example). But people's tolerance varies widely. There are plenty of people who would place S/M in the same category as child molesting, on the logic that, by their nature, such acts cannot be fully consensual. (Again, I personally disagree if all involved are adults.) It's not unreasonable to assume that many of these people are therapists and would feel compelled to address the perceived aberrance whether it was distressing the client or not.
Besides, if the idea of S/M doesn't offend you enough to impel you to interfere, it's just a matter of raising the bar on the example until I find something kinky enough. :-) Instead of a sadomasochist, what if your client was a necrophiliac who had no distress over that and had come to you for other reasons?