I agree the Catechism definition is pretty vague and could encompass all kinds of works accepted as having significant artistic value. Sex is part of human life and it’s a bit unrealistic to expect humans to never depict it, suggest it or write about it at all.
The legal definition of obscenity/ pornography in USA is usually some work that is made for the primary purpose of sexually stimulating the viewer or reader of the work, and thus lacks significant artistic value. One might also make an exception if the work has historic, anthropological or other scholarly value and is being approached in a scholarly way. For example, someone who is researching Chinese culture might be interested to see examples of what was considered pornographic in China in the 1700s.
I think when it comes to individuals viewing or reading something, as always we need to look at the reason they’re doing it and how they’re affected by it. If someone is reading a book as source material for a scholarly paper, that’s way different from reading a book to get yourself sexually aroused. If a movie is actually focused on telling a story about medieval knights and there happens to be a 2 minute sex scene in the middle of the film, it’s probably easy enough for most viewers to ignore the scene or skip over it. If on the other hand the sex scenes were 80 percent of the film, then it’s probably not something you want to watch.
The reason why 50 Shades of Grey is considered pornography is largely because the writing is terrible and because it came out of the world of fanfic, which has a huge porn subculture of people writing, drawing and generally making art to excite themselves and other people in the fandom.
), but keep in mind that there’s been widespread criticism of both the books and movies for their glamorization of violence against women. 