" 'God made me pregnant' sounded a tad more likely in the old days before our models of the world contained (quotations of) Y chromosomes. "

I don't know about that; the whole point about the "virgin birth" was that it was miraculous, i.e. physically impossible. Had they known about DNA, the story would have included God creating some DNA for "his" side of the deal. Saying that knowledge of DNA would have made the virgin birth less believable is like saying greater knowledge of classical physics would have made people more skeptical of Jesus walking on water. Impossible == Impossible.