Erotic romance is erotica written to women’s tastes using the conventions of the romance genre. It’s a spinoff from the romance genre, and is heavily influenced by slash and mainstream erotica. (In fact, with the slow death of traditional erotica presses like Blue Moon and Black Lace, erotic romance and erotic romance-influenced women’s erotica is taking the place of older types of erotica.)
What are the conventions of the romance genre?
Part of it is the style–lusher, purpler than mainstream erotica–that’s ported straight over from romance. But there are also Rules:

Once the …

Once upon a time there was a genre called Romance; and many women did read it, the young and the old together. And there was much enjoying of descriptions of swells of waves and rising doves and sheets of raw sensation, and many fadings to black.
But the hearts of some women were troubled. “Lo,” said they, “we do live in the 21st century, yet our stories of that which passes between a man and a woman involve exceeding numbers of waves, and doves, and other things which are not meet …