Robert Price (Incredible Shrinking Son of Man) has alerted me to an interesting possibility that Matt and Luke (that is, the original authors) may not have intended that Jesus had a miraculous conception. Mark never gives any hint that he was aware of any birth tradition surrounding his Jesus, Matt and Luke have generally been considered the first to incorporate this element into the legend. However Price and Jane Schaberg (Baba pg.116) have reexamined the texts and come to a different conclusion.
In their opinion, Matthew does not yet know of the virgin birth legend. He is much concerned with the irregular birth of Jesus, it is true, but he means only to make the best of a tradition, without denying it, that Jesus was the son of Mary and someone other than Joseph, perhaps a Roman soldier named Pandera ("the panther," in fact a widely attested name/epithet of Roman legionaries), as Jewish polemic always claimed. Presumably Mary would have been raped, possibly seduced, by this man.
Schaberg asks why Matthew should include only four female names (actual or implied) in his genealogy of Jesus: Tamar (1:3), Rahab (1:5), Ruth (1:5), and Bathsheba (1:6, "her of Uriah"), and all of these sullied by reputations of sexual indiscretion. Tamar posed as a harlot in order to trick old Judah into giving her a child, her due by the custom of Levirate marriage (Gen. 38). Rahab was a Jericho harlot who sold out her people and sheltered the two Israelite spies (Josh. 2), and later, according to tradition, married no less than Joshua himself. Ruth appears to have seduced her kinsman Boaz, climbing into bed with him after the orgiastic harvest festival (Ruth 3:6-10 ff.). Bathsheba was the wife of King David's lieutenant Uriah the Hittite, whom David betrayed and disposed of in order to have Bathsheba for himself (2 Sam. 11). Jewish tradition, copiously quoted by Schaberg, honors all four women despite their dubious morals, because in each case God brought good out of their sin. These women all become foremothers of David, the sun-king Solomon, and the Messiah.
Various Targums (Aramaic paraphrases of the Hebrew text, incorporating popular interpretations current at the time, like Kenneth Taylor's Living Bible today) of Genesis have the voice of God sound forth to prevent the stoning of Tamar. "It is from me that this thing comes" (Targum Pseudo-Jonathan). "Both of you are acquitted at the tribunal. This thing has come from God" (Fragmentary Targum). "They are both just; from before the Lord this thing has come about" (Targum Neofiti I). "When Judah said, 'She is righteous,' the Holy Spirit manifested itself and said, 'Tamar is not a prostitute and Judah did not want to give himself over to fornication with her; the thing happened because of me, in order that the King Messiah be raised up from Judah'" (Midrash Ha-Gadol I, a medieval commentary). Schaberg suggests that this is what it means when Matthew has Mary vindicated in Joseph's eyes by the word of the angel, "That which is conceived in her is of the Holy Spirit" (1:21). That is, according to the will of God.
And the reference to a "virgin" conceiving? Schaberg observes that the Greek parthenos must have picked up the same sexual ambiguity as the Hebrew almah, since the Septuagint translators would have had no reason to understand it in the context of Isa. 7:14 to imply technical sexual virginity. Perhaps Matthew didn't intend the technical sense, either. His answer to the "Jesus ben Pandera" slur would have been to take the bull by the horns and make the best of it, as Christians had once done in the case of Jesus' lack of Davidic credentials. The only miracle Matthew had in mind, on this reading, was that of the providence of God whereby the bad may be turned to good.
There are quite different reasons to wonder if Luke had actually intended to tell a story of a virginal conception. Here the argument is text-critical. There is a stray manuscript (Old Latin manuscript b) that omits Mary's question in Luke 1:34, "How shall this be, since I know not a man?" If this verse were in fact not part of the original text, and in fact the text we have has been fleshed out with additional narrative, it would make better sense of the passage. For one thing, the words of Gabriel, like his recital to Zechariah (Luke 1:14-17) and those of Zechariah (Luke 1:68-78), Mary (Luke 1:47-55), and Simeon (2:2-35), would then proceed uninterrupted by prose insertions.
For another, verse 34 makes Mary counter the angel with a skeptical objection precisely parallel to Zechariah's in 1:18, "How shall I know this? For I am an old man, and my wife is advanced in years." Gabriel strikes him deaf and mute until the child John is born, in punishment for daring to doubt his word. Would Luke so easily attribute the same incredulity to Mary, and if he did, would he let her off with no angelic reprisal? Note that without this verse there is nothing in Luke that even implies a supernatural conception or birth. Everything else anyone says about the Nativity simply concerns the great identity and destiny of the child Jesus. For the angel to tell Mary that she will at some future time conceive a child would be nothing to doubt as Mary does in verse 34. She is after all betrothed to be married! The very unnaturalness of the question implies again that it is a clumsy invention designed precisely to inject the foreign notion of a virginal conception. And if this is so, the whole notion of the virgin birth enters Luke's gospel by the way of later scribal alteration, to square the text with the emerging doctrine of the virgin birth.