I think you're right, there, with the comparison to published authors. I think that there is, at least in America (the only country I'm arguably qualified to judge), a bias or pervasive belief that engaging in sexuality that's out-of-the-norm can only lead to Very Bad Things and/or Emotional Misery. And I think that bias or belief is strongly reflected by what the publishing houses will publish -- they will only publish things that they think will sell. So we end up with a slew of stories that all sort of deal with dark!sex in the same, kind of one-dimensional way.

And, oh!, there's another bias/belief that says it's not ok to care about/love/have reallly good sex with people that are amoral or otherwise not good.

So for me, I like dark!fic because it often addresses these two biases/beliefs in what I feel are more realistic or more human ways. Wow! I had an actual thought, there! This is an interesting topic - thanks for posting it!