A good read for this tail-end of the Christmas season is this essay by NT Wright dealing with the birth narratives in Matthew and Luke.  "Suspending Scepticism:  History and the Virgin Birth" examines the interplay between the worldview of the gospels and that of the post-Enlightenment West.  Wright deftly places the virginal conception of Jesus in proper context, certainly not denying it but subordinating it to his bodily resurrection and divinity.

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...Some things must be put in a "suspense account"--in Marcus Borg's happy phrase--while others are sorted out.  The birth narratives have no impact on my reconstruction of Jesus' public agendas and his mind-set as he went to the cross.

...Because I am convinced that the creator God raised Jesus bodily from the dead, and because I am convinced that Jesus was and is the embodiment of this God...my worldview is forced to reactivate various things in the suspense account, the birth narratives included.

There are indeed more things in heaven and earth than are dreamed of in post-Enlightenment metaphysics.  The "closed continuum" of cause and effect is a modernist myth.  The God who does not "intervene" from outside but is always present and active within the world, sometimes shockingly, may well have been thus active on this occasion.

...There is no pre-Christian Jewish tradition suggesting that the messiah would be born of a virgin...

The only possible parallels are pagan ones, and these fiercely Jewish stories have certainly not been modeled on them.  Luke at least must have known that telling this story ran the risk of making Jesus out to be a pagan demigod.  Why, for the sake of an exalted metaphor, would they take this risk--unless they at least believed the stories to be literally true?