A scenario is okay, but enthusiastic and attractive performers are paramount. I’ve seen a few porn movies with a passable plot and halfway decent acting, like “A is for Asia,” but I’d be hard pressed to remember what actually happened in them. Hot scenes from those movies, on the other hand…
I think one reason that most people prefer less plot is not only bad acting or bad writing, but also because building up characters makes them less generic. It’s harder to use that person as a proxy for yourself, and that’s what most people — particularly men — do when watching porn.
Women usually want stronger identification with a character than men do, or more details so that they can place themselves in the fantasy. That depth of setting is probably the main reason that women prefer written erotica. Even in woman-friendly porn, there’s usually very little depth to the characters regardless of how much more attention the film makers lavish on the setting, mood, and costuming, and it’s for the same reason; it’s harder to identify yourself with one of the characters if there’s too much substance there. Since written stories are usually told from the point of view of a single character, and in erotica she may or may not even have a name, all they have to do is replace the character with themselves.