Why?
We'll get to specific godmen with divine fathers and mortal mothers in a minute. But first I want you to understand why ancient religion used the divine-father-mortqal-mother myth over and over. Why did they? Because it fit exactly with how the ancients understood divinity.

The ancients faith was greater than ours. They didn't believe in sissy spiritual Gods. Their Gods were real -- real physical beings who lived in the physical universe we live in. Where did the Gods live? Up there.

Now this Apis, or Epaphus, is the calf of a cow which is never afterwards able to bear young. The Egyptians say that fire comes down from heaven upon the cow, which thereupon conceives Apis. The calf which is so called has the following marks:- He is black, with a square spot of white upon his forehead, and on his back the figure of an eagle; the hairs in his tail are double, and there is a beetle upon his tongue.
[Herodotus 3.28]

Alexander the Great
Alexander (he died in 332 BC, but no one knew. It took a long time for folks to realize what year it was. This just goes to show how much smarter we are than the ancients) was the son of the king's wife Olympias, and the God Zeus Ammon, conceived by a thunderbolt. >>

(Kind of reminds you of Herodotus and Apis, huh? Funny, you never hear believing scholars bloviate on how the stories of Apis' and Alexander's births developed independently. I wonder why that is.)

Mr. A. Great

Maybe. There was also this story about Al's mom and a divine snake.

Plutarch even goes on to give a third version of Alexander's divine parantage -- I won't bother you with that one. You're welcome.

[2] ... It is said that his father Philip fell in love with Olympias, Alexander's mother, at the time when they were both initiated into the mysteries at Samothrace.. . . On the night before the marriage was consummated, the bride dreamed that there was a crash of thunder, that her womb was struck by a thunderbolt, and that there followed a blinding flash from which a great sheet of flame blazed up and spread far and wide before it finally died away. . . .[The soothsayer] Aristander of Telmessus . . . declared that the woman must be pregnant. . . At another time a serpent was seen stretched out at Olympias' side as she slept, and it was this more than anything else, we are told, which weakened Philip's passion and cooled his affection for her, so that from that time on he seldom came to sleep with her. The reason for this may either have been that he was afraid she would cast some evil spell or charm upon him or else that he recoiled from her embrace because he believed that she was the consort of some higher being.

[3] ... According to Eratosthenes, Olympias, when she sent Alexander on his way to lead his great expedition to the East, confided to him and to him alone the secret of his conception and urged him to show himself worthy of his divine parentage. . .
[Plutarch, Life of Alexander, Chapters 2 - 3]

Rome's founder, Romulus, was the Son of the God Mars, and Rea Sivia, a mortal Vestal virgin

Adding crime to crime, he murdered his brother's sons and made the daughter, Rea Silvia, a Vestal virgin; thus, under the presence of honoring her, depriving her of all hopes of issue.

[1.4] But the Fates had, I believe, already decreed the origin of this great city and the foundation of the mightiest empire under heaven. The Vestal was forcibly violated and gave birth to twins. She named Mars as their father, either because she really believed it, or because the fault might appear less heinous if a deity were the cause of it.
[ Livy, History 1.3 - 4]

Then, when a few men gave the lead, they all decided that Romulus should be hailed a god, son of a god, king, and father of the Roman state. And in prayers they begged his grace, beseeching him to be favorable and propitious towards them and ever to protect his descendants.
[Livy, History 1.16]

The first Roman emperor Augustus (62 BC - 14 AD), was the son of the God Apollo, conceived by a holy-snake.

When Atia had come in the middle of the night to the solemn service of Apollo, she had her litter set down in the temple and fell asleep, while the rest of the matrons also slept. On a sudden a serpent glided up to her and shortly went away. When she awoke, she purified herself, as if after the embraces of her husband, and at once there appeared on her body a mark in colours like a serpent, and she could never get rid of it; so that presently she ceased ever to go to the public baths. In the tenth month after that Augustus was born and was therefore regarded as the son of Apollo.
[Suetonius, Life of the Deified Augustus, Chapter 94]

Publius Cornelius Scipio 'Africanus', The Elder (236 - 184 BC)

"From the sun rising above the marshes of Maeotia
There is no one who may be equal in deeds.
If is it right for anyone to rise into the regions of the gods,
For me alone the greatest gate of heaven stands open."

Epitaph of Scipio Africanus - Q. Ennius

It is recorded that the mother of Scipio Africanus, the elder, had the same experience as Olympias, Philip the Great's wife and Alexander the Great's mother,... his mother had long been believed sterile and that Publius Scipio, her husband, had despaired of having children. Then, while her husband was away and she was sleeping on her own, a huge snake was seen beside her, in her room and in her bed; when those who saw this snake shouted out in terror, it vanished and could not be found. Scipio consulted the harupices about this and they held a sacrifice and gave a response that children would be born. Not long after the sighting of the snake, the woman began to show all signs of being pregnant; in the tenth month, she gave birth to this Publius Africanus, the man who defeated Hannibal and the Carthaginians in the Second Punic War. But it is much more because of his achievements than because of that prodigy that he also <i.e., as well as Alexander> is thought to be a man of godlike quality.
[Aulus Gellius, Attic Nights VI. 1.1-6, 2d century AD]

The godman Dionysus was the Son of Zeus and the mortal Semele.

I am Dionysos, the son of Zeus,
come back to Thebes, this land where I was born.
My mother was [the king] Cadmus's daughter, Semele by name,
midwifed by fire, delivered by the lightning's blast.
And here I stand, a god incognito, disguised as a man.
[Euripides, The Bacchae, v 1 - 5 (5th century BC), -- which you can find in: Meyer, Marvin W.. The Ancient Mysteries; A Sourcebook of Sacred Texts (1987), pg. 67]

And Semele, daughter of Cadmus was joined with him [Zeus] in love and bore him a splendid son, joyous Dionysus,--a mortal woman an immortal son. And now they both are gods.
[Hesiod, Theogony 940, c. 8th century BC]

Pausanias describes the birth of the God Attis:

a daughter of the river Sangarius, they say, took of the fruit and laid it in her bosom, when it at once disappeared, but she was with child. A boy was born, and exposed, but was tended by a he-goat. [Pausanias, Description of Greece 7.17.9-11]

The second century Church Father Origen says of the Jesus' virgin birth,

He lists a number of Pagan Gods born of virgins: Danae, Melanippe, Auge and Antiope. The stories about these Gods are "ancient," says Origin, but unlike the story of Jesus' virgin birth, only fables. [Origin, Against Celsus 1, 37]

"We [Christians] are not the only persons who have recourse to miraculous narratives of this kind." [Origin, Against Celsus 1, 37]

The second century Christian Justin Martyr says of Jesus,

"He was born of a virgin, accept this in common with what you believe of Perseus." [Justin Martyr, First Apology, 22]

"The devils...craftily feigned that Minerva was the daughter of Jupiter not by sexual union." [Justin Martyr, First Apology, 64]

POCM is about history, not ethics. Jesus' miraculous conception was not new and unique -- that's important for the historical truth, or not, of gospel claims.

Other people take the point farther, and ask: "Can we morally say: "Ours is history, yours is a lie"?

Here's how Mrs. Crossan's little boy Johnny puts it:

Augustus came from a miraculous conception by the divine and human conjunction of [the God] Apollo and [his mother] Atia. How does the historian respond to that story? Are there any who take it literally?... That divergence raises an ethical problem for me. Either all such divine conceptions, from Alexander to Augusts and from the Christ to the Buddha, should be accepted literally and miraculously or all of them should be accepted metaphorically and theologically. It is not morally acceptable to say...our story is truth but yours is myth; ours is history but yours is a lie. It is even less morally acceptable to say that indirectly and covertly by manufacturing defensive or protective strategies that apply only to one's own story. [John Crosssan, The Birth of Christianity, 1998, pg 28 - 29.]