This thread once more showed me how lucky i am to not believe in anything. Life's just so much easier when you're an ignostic atheist
To the original question (which i think is a good one): Nope, BDSM is not a religion, because unlike the only religion i know (catholicism) and most i heard of nobody tries to tell me what is right and wrong, good or bad, allowed or forbidden.
I can walk through the world of BDSM, pick things i like, reject ideas i dislike, mix it all together and form something that perfectly suits my needs.
If i'm really lucky, i can do that with someone else, or more than one, and nobody tells me with whom and how many of them.
Now which religion allows you that many choices?