Constant Lambert's ballet Tiresias, recently recorded on Hyperion,
probably fits the bill.  Quoting from the booklet: "...  not a likely
ballet scenario, with the stage requirement of the lead character changing
sex, two snakes copulating, an argument about which of the sexes enjoys the
sexual act the most, a downbeat ending, and a convoluted story not familiar
to the general public."

-- Art Scott
Livermore, Cal.