I'm the first to say I'm no expert on erotica...take this with a grain of salt.
But it has come into my mind that there are certain strong similarities between horror and erotica.
Specifically, horror and erotica are genres of emotion. A writer of erotica has as her ultimate goal instilling the emotion of arousal.
Erotica is written to titillate, and sometimes to fulfil. Everything that happens in an erotic piece is focused towards that one goal...although if there isn't at least some plot, you're writing pornography. Erotica is about desire.
Horror is written with the specific emotion of fear in mind. The goal of the horror writer is to...well...horrify. To induce a controlled, cathartic state of fear. It's not dissimilar to the goal of designing a roller coaster, really. The reader wants to be frightened, whilst knowing he is safe.
In both cases, the primary goal of the writer is a pure, raw emotion.
Meanwhile, if one is writing a western, one's goal is quite different. Westerns, science fiction, fantasy, historical. All of these are genres of setting. The place defines the genre. The reader becomes, in this case, an explorer.
Crossing between the two...between western and romance and fantasy and horror brings these things together. I think it might be a worthy exercise for writers of 'setting' genres to dip into the emotional genres in order to hone the ability to write for emotional effect.