I have two thoughts.
1) Cheating Husbands. We have "Wife Lovers" but not one about husbands who cheat.
2) Sci-Fi. Now, I know for this category it might require Lush to allow "non-human" characters to have sex, but it's just a thought.
Big-haired Bitch/Personality Hire
I think the categories are fine. Anything that isn't covered by the categories is covered by tags that can be searched, and these tags are in and of themselves subcategories. If every little thing were listed in its own category, we'd be scrolling the homepage forever. It would just be a clusterfuck of categories, and it seems a bit counterproductive.
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How about specifying the nature of the bisexual stories. Within the bisexual category I'm really only interested in those featuring MMF activity.
Improvements of interracial stories?
only problem with a humor category would be the subjective nature of the topic...how many times did some send you something you thought was really funny, and you go, "huh?" Hell to this day, I still don't understand why my friend keels over into helpless laughter over Napoleon Dynamite.
As for a category for good grammar stories, that might be unfair because you would drive the teachers in the group nuts if they found someone's participles dangling in the good grammar section.
College stories, true stories.