Wright on the Virgin Birth:

We would have to suppose that, within the first fifty years of Christianity a double move took place: from an early, very Jewish, high Christology, to a sudden paganization, and back to a very Jewish storytelling again.

The evangelists would then have thoroughly deconstructed their own deep intentions, suggesting that the climax of YHWH’s purpose for Israel took place through a pagan-style miraculous birth.

To put it another way: What would have to have happened, granted the sceptic’s position, for the story to have taken the shape it did? To answer this, I must indulge in some speculative tradition-history. (Bear with me in a little foolishness.)

This is how it would look: Christians came to believe that Jesus was in some sense divine. Someone who shared this faith broke thoroughly with Jewish precedents and invented the story of a pagan-style virginal conception. Some Christians failed to realize that this was historicized metaphor, and retold it as though it were historical. Matthew and Luke, assuming historicity, drew independently upon this astonishing fabrication, set it (though in quite different ways) within a thoroughly Jewish context, and wove it in quite different ways into their respective narratives. And all this happened within, more or less, fifty years.

Possible? Yes, of course. Most things are possible in history. Likely? No.

- Suspending scepticism: History and the Virgin Birth – Opinion – ABC Religion & Ethics (Australian Broadcasting Corporation)