The virgin birth works for me in 2005 - it's a basic of the faith.
That fact is we don't really know the details of how Jesus was conceived. That's generally the point. We are simply incapable of understanding the works of God and to say we understand how women *usually* are impregnated is irrelevant. It seems Spong merely assumes God is impotent (unintended double meaning) and also that He does not act in the world, therefore Jesus could not have been conceived by God, whatever the actual molecular mechanics might have been.