Literature has always had plenty of sex, but we’re adults here, bodice-rippers are pornography for women. And that’s cool.
But why can’t I check out a copy of Penthouse Letters at the library? Why aren’t they for sale in the grocery store checkout lane?
Perhaps because one type is fiction and the other is “real”. Although I’m not sure how real some of those letters are.
Pictures.
First, I think there’s an important distinction between “obscenity” and “pronography” or sexually explicit material. As I understand it, obscenity is material that, in the eyes of the person labeling it as such, is inherently morally repugnant. Pornography or sexually explicit material is certainly inappropriate in some contexts but may be deemed acceptable given the right context or purpose.
Second, some romance novels are of course perfectly chaste, and of those that do contain sexually explicit passages, those passages are there to serve the story and contribute to the overall effect. With genuine pornography (e.g. Penthouse Letters), any story there may be is there to serve the sex, and aid the reader’s arousal. This may be true of some “bodice rippers” as well, but it’s harder and less obvious to know which side of the line they fall on.
Also, a good Romance novel has plot, characterization, narrative description, some engaging scenery, some dramatic conflict, a case of minor characters, etc.
Penthouse letters were about ten per cent set-up and ninety per cent boffing. They were very scanty in terms of any literary quality.
(Both genres are hugely stereotyped. Both can be written by computers.)