NYU Press

“As a part of the holy trinity, Jesus was regarded as divine, and in predominant Christian belief this divinity was not compatible with human copulation. So Mary was a virgin, with the baby Jesus implanted by divine intervention.
This was of course a marked departure from other religions in the classical
world that had not ventured such a complex statement about divine presence among mortals, and that had often been quite comfortable with the idea of sexual exploits among the gods and as sources of other gods.”
― Peter Stearns