For all of ancient civilization, there is a creation story and invariably many stories about virgin births. Virgin births: the births of gods, demigods and great men who will become very famous later on. The common denominator of these virgin births is that there is no sexual intercourse between a man and a woman for the inception of this child. In this scenario, there is some kind of an implicit taboo about sexual intercourse in vaginal births. It is imperative that the woman who gives birth to the precious baby should remain a virgin except in very rare circumstances.
Let me start out with Greece. There are hundreds of virgin births in Greek mythology. Just to name a few, let us examine the birth of Athena, who is the patron goddess of Attica. Athena was divinely conceived within Zeus and when the labor pains came, Zeus experienced intense headache because the baby was growing in his brain. Somebody came with an axe and cut open his skull and took out the baby. There came Athena in glowing colors, with a helmet, breast plate and a sword and all. In this case, there was no sexual intercourse and the virginity of Zeus (who is a man) is protected. Later on when Athena wanted to make a child to be the first King of Attica, she divinely conceived a baby in her thighs and later on the thigh broke open and the baby came. His name was Erecthonius. Here Athena's virginity is maintained, her uterus or vagina was not touched, and of course she did not have sexual intercourse. In modern medicine we call this type of embryo in the thigh, a 'teratoma'. But, of course, there is no point in arguing reasoning in mythology.
In Egyptian mythology, the high gods Isis and Osiris were brother and sister. Now Isis wanted to make a child. She conceived the baby within herself without any sexual intercourse and the baby was born as a virginal birth. Osiris was both her husband as well as her brother. She could have had intercourse with him but she did not bother him. Her virginity was preserved. In Zoology we call this type of reproduction 'parthenogenesis'.
Now when we get back to India, there are many stories in fact. The first and foremost is the birth of Lord Krishna. Krishna was divinely conceived in the uterus of Devaki and in due course, Devaki had a vaginal birth which is an exception to the rule. But, of course, Devaki was not a virgin. She had six vaginal births before, but the conception of Krishna was divine. In Indian mythology, there is a very prominent character called Kundhi. She is the mother of the Pandavas. The Pandavas are the righteous people in the epic war of Mahabharata. Kundhi got her first child from the celestial body sun and she conceived the baby in her thigh and from her thighs came out a legendary hero called Karna (Karnan). Kundhi had a few other kids, too; all of them without any sexual intercourse and without any vaginal deliveries.
The story of a divine birth gradually transcended to humans, too. For example, there are many great men who were 'said' to be born without sexual intercourse but by a divine conception in a female body. Such stories are prevalent about the Greek philosophers: Pythagorus, Socrates, Plato and Aristotle. Even for Alexander the Great, there is a tradition that he was a divine baby. Similar stories of divine conception are also prevalent about the great Indian philosopher Buddha. Some people have contested that he was another incarnation in the Hindu pantheon.
The story of Jesus is also not much different. Here it was a divine conception. A man of great significance who is the son of God, who is supposed to be ascended to heaven cannot be born via a sexual intercourse of a mortal man and a woman, not at all. The people who wrote the testaments had to write such a divine story because that was the common practice around that time. As I have stated earlier on, there is a taboo about sexual intercourse; something inferior, unholy and therefore must be avoided when it comes to the birth of noble men.