The culture of s&m isnt about sex (or only about sex), what individuals do with it is a whole 'nother story. Too many people watched or read 50 shades of nonsense and think that's what s&m is. Sunstone is a more accurate depiction.
Also what KittyCatOmaniac said.
Just have to read "Nana to Kaoru".
There's absolutely no sex in it (lots of service though), but it's damn thrilling. One of the cutest dom/sub relationship I ever read about.
There's no sex in the sense of going all the way, but it's clearly about sexual tension, about desire; they will get to sex sooner or later (not necessarily in the manga, but you know their relationship is headed in that direction). The SM in Nana to Kaoru is thoroughly sexual. If that's the kind of yardstick people are using for "Oh, no, no, SM isn't a sex thing" I am greatly skeptical.
And I think it's unnecessary, a sort of defensive reaction, taking on board a hint of puritanism from the critics, like "Oh, no, it's not some mere tawdry sex thing"--hello, sex is a foundation of our existence as biological organisms, we wouldn't be here without it as much as we wouldn't be here if we didn't eat. Obsession with sex is a basic important survival trait, so of course sex is intertwined hugely with our cultures, with huge amounts of what we do. Back to SM, if the point isn't driving that fundamental obsession (in a sometimes somewhat obscured way) then I don't get the point; I don't see how sex is just an optional extra. I can envision SM without a sexual component, I guess, but it seems like the drives involved must be much less basic.
(Although I suppose Tachi's reaction to SM is almost nonsexual, it's like a different kind of athletic effort for her. But I don't really grok her take on it for precisely that reason.)
Well, maybe I can properly explain what's going on.
Sadism is the pleasure received when being aggressive towards someone. Kinda like the feeling you have when you want to squeeze a cute animal. Different, but similar. Masochism is the pleasure received when getting aggresived on, if that makes sense. Together it works very well and can become a very selfless relationship: you act as a sadist in order to make your partner feel good, and seeing that in turn makes you feel good. Vis-a-vis for masochism. (fun fact: existentially all babies are technically masochists...but that's a whole 'nutha discussion)
However, it's not necessarily a selfish desire-fulfilment. Harken to the scene where Yuuka commented how seeing Moe was upset about her music player being broken but didn't feel particularly good about it. She doesn't want to see Moe in actual pain. She herself wasn't in tune with her sadist side until she met Moe. There hasn't really been any predominantly sexual themes until this chapter. It's been mostly romantic.
Essentially S&M is about "i like to push my partner" and "i like to be pushed by my partner". It's a cyclical and lovely thing. I can't speak with complete ethos, of course, because I am only partially a masochist. But I can say that the main reason we can see a lot of their dynamic as sexual is because Occidentally (and partially Orientally, but far more for us) we are taught that sex and love are completely interlinked, much more so if physical contact is involved.