"The ceaseless intermingling of Heaven (Yang) and Earth (Yin) gives form to all things. The sexual union of male and female fives life to all things. The sexual union of male and female is thus regarded as the essential earthly manifestation of the great cosmic dance of Yin and Yang.
A commentary in the Book of Changes states, "The interaction of Yin and Yang is called Tao, and the resulting generative process is called change." Therefore the Way may be found in the interaction of Yin and Yang, in the union of opposites, not in one extreme or the other. Because of the ceaseless flux of Yin and Yang, the primary attribute of the Way is constant change, not stasis.
In the Book of Changes, the hexagram that symbolises sexual union is the 63rd, called "Completion". It consists of the trigram for "water/woman/clouds" placed over the trigram for "fire/man/light". This not only places Yin on top of Yang, it also suggests the image of cauldron water slowly coming to a boil over a fire. This is the quintessencial Taoist image for human sexual intercourse, concisely symbolising the essential differences between men and women in the sexual act. In order to last long enough to bring that cauldron of water to a rolling boil, the man must ration his fuel and carefully control his fire. If he burns his fuel too fast, his fire expires prematurely, leaving the water only lukewarm. But if he conserves his fire long enough to bring the water to a boil, then even the smallest of flame suffices to keep it hot for a long time."
The Tao of Health, Sex and Longevity by Daniel Reid
