Echoing the above comments, I don't think there's a perfect way to do it. Sure, Strap-On Sex arguably belongs under Toys, and Horror and Monster Sex seem an unnecessary dichotomy ... and where's the split between those and Supernatural? Also, why have the vague "Occupations" when most of those stories could either go under Uniform, Medical or Office Sex?
And all sex involving four or more people is supposed to be in Group Sex? That can include orgies (have written that), gangbangs (just wrote that), circle jerks (ditto), bukkake (story planned) and swinging (just wrote that too, although Swinging gets its own genre for some reason), all of which have separate appeal.
Then again, I love that Cuckold and Wife Lovers are separate, because even though the physical acts tend to be about the same, the emotional dynamic can be vastly different. Not to mention the appeal: Cuckold and its related degradation are a pretty hard turnoff to me, while Wife Lovers/hotwife action is a pretty hard on.
In any event, these are the categories we have, and for which hundreds of authors have intentionally written stores. Like Kimmi said, maybe the ambiguity of the categories is a good thing. God knows I wouldn't have much to write about if my own sexuality was cut-and-dried.
