Although it is usually used to indicate "adult content," it's actually based on a genetic mutation. There are two genes that determine gender: X and Y, and two slots for them: YY doesn't exist, XY is male, and XX is female. The mutation when a fetus gains 3 X genes is dubbed "super-female" and is characterized by the subject having engorged "Asspects" and being infertile; a perfect candidate for "adult-themes."