1) there are substantial differences between the narratives of Jesus’ birth and those of pagan births involving pagan deities that include but go beyond the virgin conception, 2) the authors of the Gospels of Matthew and Luke were aware of the pagan birth stories involving deities and sought to distinguish Jesus’ birth from them, and 3) the efforts of Matthew and Luke to distinguish Jesus’ birth from rival pagan accounts help explain why some early Christians did not highlight the virgin birth of Jesus in their preaching and writing.
I would add the following points to the many good observations he makes in the two articles.
Ben Witherington comments that "most scholars" think that the infancy narratives are more like Jewish infancy accounts than pagan birth legends (in Joel B. Green, et al., edd., Dictionary Of Jesus And The Gospels [Downers Grove, Illinois: InterVarsity Press, 1992], p. 60). Darrell Bock writes that there’s a "consensus" among scholars to reject the view that the virgin birth was derived from pagan mythology (Luke, Volume 1, 1:1-9:50 [Grand Rapids, Michigan: Baker Books, 1994], n. 4 on p. 103).
Even a scholar as generally skeptical of the infancy narratives as Raymond Brown wrote:
"If the marital situation between Joseph and Mary [portrayed in Matthew's gospel] were not a fact and could have been created according to the dictates of Christian imagination, it is difficult to see why a situation less open to scandal was not contrived. For instance, instead of picturing Mary as already pregnant, the narrator could have imagined her as betrothed to Joseph but without child. Then he could have had the angel of the Lord appear and begin his message with 'Joseph, son of David, hasten to take Mary your wife into your home.' Everything else in 1:20-25 could follow, and there would be no hint of scandal....Matthew's world view and that of his opponents is not one in which deities have sexual relations with men or women and beget children. He is in confrontation with Pharisees and in his account of the ministry he is most careful not to give them anything they can use against Jesus (e.g., his omitting the spittle miracle narrated in Mark 8:22-26). If the situation described in Matthew is not a factual one but is the product of Christian romantic imagination, one must deem it a great religious blunder; for it gave rise to the charge of illegitimacy against Jesus that was the mainstay of anti-Christian polemic for many centuries....One may hypothesize that independently Matthew and Luke hit upon the pattern of an annunciation, the idea of a virginal conception, etc.; but it is more plausible that these are earlier ideas that each has taken over and developed in his own way. I find totally implausible that they would independently chance upon the same peculiar marital situation as a setting for the annunciation....Leaving aside formal biographies, one can make a better case that even Jews would have known (sometimes derisively) popular stories about the gods, but would they have wanted to imitate them in describing the Son of the Lord God of Israel? Most lines in the infancy narratives have patent OT parallels; it is very difficult to show that the evangelists drew upon the proposed and far more distant Greco-Roman parallels. The two evangelists could have written their infancy narratives without ever having heard or read biographies and tales composed by pagan writers; the orientation of the Gospel narratives could have come from Hebrew or LXX forms of the biblical stories of the Patriarchs, Moses, and David (enlarged by subsequent oral lore), plus some Jesus tradition and theological reflection....Two mutually hostile traditions about Jesus' birth, Christian and Jewish, came to agree on that point [that Joseph wasn't Jesus' biological father]. The Christian claim that Joseph was not the father (Matt, Luke) can scarcely have arisen by reaction to Jewish calumny - that would have been answered by saying Joseph was the father - so that, unless one wants to say that Jewish polemic about Jesus' illegitimacy was based entirely on misunderstanding, it helps to show that Christians were claiming an unusual conception....On 528-31 above I argued that although the limited NT evidence is not conclusively probative, to posit historical fact as an explanation of Matt's and Luke's agreement on the v.c. [virginal conception] is more conformable to the evidence than to posit fictional creation....[quoting another source] 'None of the proposed parallels [to the virginal conception], either pagan or Jewish, seemingly accounts for the story we find in the NT.'" (The Birth Of The Messiah [New York, New York: Doubleday, 1999], pp. 142-143 and n. 28 on p. 143, n. 41 on p. 247, 579-580, n. 318 on p. 703, 705, 707)
Celsus, a second-century non-Christian Gentile who consulted Jewish sources for information on Christianity, rejects the virgin birth account, as we would expect. But he attributes the virgin birth claim to Jesus Himself (Origen, Against Celsus, 1:28). The timing of the gospels and their sources suggests that the virgin birth claim was circulating when close relatives of Jesus were still alive. The claim may have been widely circulating even prior to Jesus' death, as Celsus suggested. If Christians for a few or several decades had been saying that Jesus was conceived by normal means, and influential church leaders like Paul had no concept of a virginal conception, as critics often suggest, why would the concept be so widespread so early on, and why would critics like Celsus and his Jewish sources think that the idea was circulating when Jesus was still alive? If it was a concept that arose in the eighties, nineties, or later, wouldn't we expect something like a half century of widespread ignorance and contradiction of the doctrine to leave more of a trace in the historical record?
The gospels of Matthew and Luke were widely used early on, as we see, for example, in Bruce Metzger's The Canon Of The New Testament (New York: Oxford University Press, 1997) and Clayton Jefford's The Apostolic Fathers And The New Testament (Peabody, Massachusetts: Hendrickson Publishers, 2006). We don't find Pauline communities rejecting the virgin birth, while a Matthean community in another part of the world accepts the concept, for example. Rather, as sources like Ignatius and Aristides illustrate, both Pauline and Matthean communities accept the virgin birth and the documents that affirm it in the earliest post-apostolic generation. Luke, Ignatius, Polycarp, and other early sources who thought highly of the apostle Paul also thought highly of the virgin birth in particular and/or the gospels of Matthew and Luke in general. Why is it that so many skeptical speculations about what Paul believed aren't reflected in the early Pauline Christians and Pauline churches?