It started out as a Twilight fanfic, which I think explains its initial popularity. Fanfic readers are, unfortunately, more apt to read stories with rape and dubious consent than anything else, and more often if it occurs in slash stories. I don't know why, but that's what I've observed over the years. Also, the author knew how to market it. I don't know of any romance writers that actually like it though. One I know compared it to the Ariel Castro case--you have a guy holding a woman prisoner and doing things to her that she only submits to because he is rich and handsome.
Every romance publisher I've looked at has specific guidelines--no rape for titillation, the hero can't be a rapist, and the heroine can't fall in love with him if he is. No more Luke and Laura General Hospital or The Flame and the Flower type stories.
In defense of romantic fiction, I will say that all genre fiction is formulaic in one way or another, if you analyze it. Someone wants something, something stops him or her from getting it, there is a struggle of some sort, either physical or emotional, sometimes both, and the thing desired is either obtained or not. It's just that in a romance there is usually a happy ending. I don't think there's anything wrong with that (obviously!), especially with so much darkness in the world; hope is not a bad thing. Yes, some romance authors have switched genres, either because of burnout or wanting to stretch their wings, but a more likely reason, imo, is because the genre is so looked down upon, and even though they're fantastic writers, they weren't taken seriously.