No it isn't. Sex is not a necessary component of BDSM, though obviously if you're a BDSM couple, it'll most likely be a component. However, let's say we have a guy who is a masochist. He might well let himself be whipped by both men and women he has no sexual relation with, because it's the intensity, the play, that counts and not his sexual attraction to the people holding the whip/flogger/cane. Same goes for people who visit professional dommes. Very often there's nothing sexual involved, they just like being forced into a dress and made to clean a room.
BDSM just isn't sexual. Sure, some vanilla people try to be kinky in the bedroom, but that's not real BDSM. And many BDSM'ers bring it into the bedroom, but they don't do that for the sex, but because they are into BDSM.
I guess it's just a lot of it seemed like you could still predict people's wants and desires based on the "vanilla" rules of dating. Maybe someone likes being whipped but they would probably prefer it to be by an attractive female (if they're a guy), or someone that seems like a suitable partner.
Plus it seems like you can fantasize about something sexual in nature while doing something else. Perhaps being whipped cause him/her to think of their body, which is a sexual thing, and in a sense another person's sexual body.
Or like why forced into a "dress?" you know? It just seems like everything is always conveniently a bit sexual if not overtly so at least in suggestion or possibility. Very rarely does it seem like people do things purely in an abstract sense without grounding in basic sexual desires....
I'm not really here to like be critical or anything necessarily, it's just something I've noticed.
For example 50 Shades of Gray I'm like.. this is sort of just the standard rom-com super rich controlling guy and submissive female that is in every other movie, like, that's the foundation, so you throw in a few whips and chains, but it's still fundamentally that sort of concept.
Maybe what I'm getting at is there isn't so much BDSM and like hardcore BDSM or whatever but just a reflection that power (equality and inequality) is the way in which everyone forms relationships, whether that's made explicit in the form of toys and such seems somewhat secondarily related.