Could it be that a sexual response is what clued most of us in to our own interests in bdsm in the first place? Most everyone can remember finding themselves surprisingly, unignorably turned on by an image or idea involving dominance - maybe it was last week or maybe it was when they were 12 - and exploring or fantasizing their way to a (probably very private) orgasm. That seems to be how it all starts for most people, and the mental connection between bdsm and sexual desire remains as we become more self-aware, seek out partners, etc.
I wonder about this a good deal. I am sexually submissive, but in my everyday life I am almost absurdly toppish. While flipping that switch in my head is very do-able in a sexual situation, and while I love to do the little things for people I care about as an expression of my affection, I have never made the leap to having a long-term relationship in which service was a feature. Perhaps I just don't have it in me, or perhaps none of my partners has happened to want that. But I feel a little bashed by reading that sexualized submission is somehow 'less than' service oriented submission. It's tough enough to find a way to walk this non-mainstream path without being told that what you feel is inadequate. Ouch.