elfs | Pornography vs. Erotica: I have the answer! (as a writer)

You are writing erotica when you write a sex scene with the knowledge that the reader is expecting the scene to reveal something important about the characters, or to solve or create an existing point of the plot, or results in one or both of the characters having a personal crisis or revelation themselves during or because of it, and that the reader will be disappointed if such a moment is not in the chapter.

You are writing porngraphy when you write a sex scene with the knowledge that the reader is expecting a sex scene, will be disappointed if one is not forthcoming soon, and will get annoyed if any of that plot stuff gets in the way of watching the characters get off.