I guess my question here would be: But if those are my characters and I designed them to be this way, and them having sex is at least a bit integral to the plot, then why should this particular slant disqualify my sex scenes from being "real" sex scenes? Isn't this sort of like saying "curry is bad and pepper is good, and too many people use curry because it's easy, so use pepper instead, you lazy fool"?

I realize you're not talking about horror sex, and you said you weren't, so believe me, I'm not taking any of this personally (and I'm also not a big Kushiel series fan either, in case you wondered). OTOH, it's just as easy to ask why there should be any sort of sex scenes at all, ever, in anything; I certainly don't necessarily read any sort of genre fiction for the sex. But if you've already laid out a society where BDSM sex is the lingua franca, there's probably going to be a lot of inequitable, negatively-charged sex going on--and maybe that author (name escapes me) isn't even thinking of those scenes as "sex" scenes per se. Maybe she's thinking of them as combat, conversation, or diplomacy.

I'm not sure where I'm going with this, but...there you are.