I've been reading some erotica lately, and in what I've read the romance is ultimately more important than the mystery or other genre. I don't just mean the SEX takes a front seat -- the romance... the relationship between the main characters... is ultimately the driving force of the book.
Is that required? If I wrote a book where there was the same amount and emphasis on sex, but there were no other romance elements (building a relationship, HEA, etc.), would readers feel like I hadn't held up my end of the bargain?
I think for a long time women's erotica was about the woman's journey as a person, that is what all the Black Lace books were like and she had sex with a lot of different people, but there were always other elements, great setting and exploration of characters that made those books great.
Then at some point it seemed Black Lace's marketing department decided that the only thing that really sold was erotic romance, the authors were told to add more romance and the words "erotic romance" were suddenly added to the covers.
Jaid Black had had great success with erotic romance with EC, it was just thought to sell more so there was a big shift to it, even in traditional publishing. But publishers like Ellora's Cave, Mischief, Carina and Samhain, Siren (not sure about Loose ID) still publish women's erotica without romance, they just don't publish as many of these books as they do erotic romance. If you wrote a book that was pure erotica you can submit to these publishers or Mischief Books it would just be labeled differently.
I think Amazon is also flooded right now with very porny cheap erotic books that are not romances or even really erotica given these really don't have much plot, and are put up in the hopes of making a quick buck, which is a separate thing again from trying to write a good quality erotic novel for women.
