The danger of writing one single sex scene lasting 20 pages is that you really need to write it very well indeed, otherwise it could become repetative and, dare I say, boring. My own personal tastes are usually to write stories WITH sex scenes in them, rather than sex scenes with a little bit of story. But if you get positive reviews for your stories then that should be a good indication of whether people like it or not. Overall, though, you always need to write stories to please yourself first, and if they also please others then that's a bonus.