Here is the last of three interesting pictures I took in the British Museum last week (the others were a prophetic print and a crystal ball):
This particular object, originating from Nepal and made of gilt copper, depicts the Buddha (in his esoteric form of Samvara) embracing his female "wisdom partner". She is holding a ritual dagger in her right hand, while he is wearing a long dangling necklace made of human skulls. Both of them are stomping on two small figures... not demons, as you might guess, but gods -- one is Bhairava (a manifestation of the Hindu god Siva), and the other is the goddess Kali.
The usual explanation for this kind of "Tantric" Buddhist imagery is that it was devised by early Buddhists as an aid to meditation. Visualizing the sex act is supposed to be conducive to the union of wisdom (symbolized by the dagger) and mental purification (symbolized by the skulls). Personally I find this explanation rather cynical (a few years ago my career as a book reviewer for the Buddhist Society came to an abrupt end when I dared to suggest this!)... but it's a pretty cool idea nevertheless. In any other religion, if you close your eyes and visualize people having sex they call you a pervert!
The fact is that Tantric visualization is quite a rare thing in Buddhism -- as far as I know it's not practiced by Zen Buddhists in Japan, or by Theravada Buddhists in South Asia. My guess is that it originally had nothing to do with Buddhism at all, but started out as a fairly standard "sex magic" practice in the Himalayan region -- then got grafted onto Buddhism when the missionaries realized it was the only way they were going to convert the locals!