Can you clarify on how BDSM is not "hurting someone"? I'll definitely give that it's not non-consensually hurting someone. I'll also give that people who are into BDSM are into different things that fall under that umbrella, and not all of them involve physical pain.

But I've also read about BDSM practices (and know people who've partaken in them) that involve getting whipped; hanging themselves from the ceiling via hooks inserted in their skin; insertion of needles, often many needles, into the hundreds, into their skin; biting so hard that bruises arise (and I'm sure some people experience broken skin), etc, etc, etc. It is undeniable that these practices cause pain.

I'm thinking that your answer may lie in the fact that BDSM is consensual - which is basically the only reason that the treatment of a person in such a manner is okay - but to be honest, I have often struggled with this being the only thing that makes BDSM okay, because after all a cutter for instance is cutting him or herself with consent, but we do not allow that in society regardless.