I for one enjoy the sex scenes AND the storyline. Though without the story, the sex scenes would be pointless fluff. :V It makes more sense to have half of the sex scenes because *Surprise!* People have sex sometimes in real life.
I think the commonly accepted reason why sex is in F/SN is because H games sell more in general than non-H, and it's easier for the non-H crowd to ignore the sex scenes than it is to attract the H crowd to make up sex scenes.