Why are games so terrified of sex?

Harley Quinn
Harley Quinn in Arkham Asylum is far more sexualised than her normal playful jester self

Ah yes, sex. Where would we be without it? Certainly, it'd be easier to find parking spaces, but much tougher to find a date to the movies (or perhaps a reason to go on a date in the first place).

Yet despite the fact that most would agree that a good horizontal rumba is rather better than, say, a good session of genocide, the number of real PC games that have dared take on more sexual subjects can practically be counted on the fingers of one hand and… oh, behave.