The Greeks called it the sacred marriage or Hieros Gamos. It is the western esoteric version of the Tantric tradition of the woman as goddess (shakti) and the man as god (shiva) uniting their energies to create something in the physical realm. In the Hieros Gamos, the woman and man are no longer considered human, their sexual union is not personal or based on sexual attraction or even love between two people. The woman channels the archtypical energy of the divine feminine and the man the archtypical divine masculine and, in the sexual act, the two energies unite in both physical vessels.
Like Tantra, the Hieros Gamos is a ritual, performed on the physical plane in order to bring both forces into the manifestation of creation. It was also a part of western alchemy as the conjunctio or union of sun and moon or king and queen and is one of the operations used in creating the Philosophers Stone or Elixir Vitae which gives eternal life. This was the reason that medieval peasants would sometimes have sexual intercourse in a plowed field. Their sexual union would hopefully provide the creative energy to ensure a fruitful harvest.
Most of what we know about the Hieros Gamos comes from Carl Jung who considered the psychic union of anima and animus, or female and male parts of the psyche as being necessary for individuation, with the divine child being the individuated person. In this aspect of the Hieros Gamos, there was no physical sex, since the union of the archtypical male and female energies occur in the person himself or herself. Jung taught that we all carry within ourselves both male and female energies. One half of the energy usually manifests in the outer world as the physical gender while the other half exists in the psyche as potential which is projected onto a partner. But it was only when the two halves of the psyche united and were brought into conscious awareness that the individual, whatever his or her physical gender, could be psychically whole.
Although banned by the Roman Catholic church, the Gnostic gospels hint at the spiritual union of Christ and Sophia, the spirit of wisdom. Sophia, so the legend went, had fallen to earth and become imprisoned in matter. It was through her sacred union with Christ that she was able to regain her former celestial status. Another forbidden teaching was that Mary Magdalene was actually the wife of Jesus of Nazareth and the fruit of their Heiros Gamos was a child who later married into the Merovingian dynasty of French kings. Thus, Mary’s womb can be seen as the Holy Grail of Arthurian legend, the vessel which bears the ‘holy blood’ of Jesus in the person of his child.
The Hieros Gamos, then, is the sacred act of sexual union performed by two persons, equal in every way, which unleashes the creative force of divine love -- the essence of true magic.