A passage based on information from this book:

“The virgin birth was believed to be the divine impregnation of a young woman by a god. The virgin mother-to-be deflowered herself be straddling the sacred lingam–the god’s erect penis–so it penetrated her. While conceiving the god’s son, the virgin placed a wreath of flowers upon the head of the god’s image. This was a symbolic act and reminiscent of the ancient Indian svayamara ceremony. The wreath symbolized the virgin’s genitals, and the head represented those of the god’s. Both the head and the lingam were anointed with holy oil in honor of the divine marriage and for the protection of the young woman during penetration. Such ceremonies were customary throughout the Middle East and Mediterranean areas. The holy oil was chrism and the god was a Christos or ‘Anointed One.’

In Roman times this ceremony was very popular with young women. They deflowered themselves upon the carved phalli of Hermes, Tutunus, Priapus or some other ‘anointed’ god before lying with their bridegrooms. Then the firstborn child was thought to be god-begotten. Thus came the phrase ‘born by the grace of God.’”

From a great site, the Mystica - check out their full topic index here.