Pursuing this theme; I am thinking about doing something extensive - and positive - about the doctrine of the incarnation, partly in response to this post, but for now I think I can do without it. So briefly: the doctrine of the incarnation is that 'the Word became flesh and dwelt among us, full of grace and truth' etc. The Virgin Birth relates to that doctrine as a 'how', not a 'what'. That is, it is an explanation of how the incarnation was possible, it is not the doctrine of the incarnation as such.
So how did this 'how' work? Here is Aquinas on the subject: "in conception the seed of the male is not by way of matter, but by way of agent: and the female alone supplies the matter. Wherefore though the seed of the male was lacking in Christ's conception, it does not follow that due matter was lacking." (ST III Q28 Art 1)
In other words, the material of humanity was supplied by Mary; the form of Christ's humanity was supplied by the Holy Spirit. Consider plasticine - it can be shaped in all sorts of different ways, yet still remaining the same substance. So too, for Christ, he was of the same stuff as the rest of us, it's just that his stuff was ordered and shaped in a different way, ie without sin, which is passed on as an 'infection' through sexual intercourse. Here's Aquinas again: "Now it was not possible in a nature already corrupt, for flesh to be born from sexual intercourse without incurring the infection of original sin."
This seems to me to be coherent as a way of explaining the 'how' of the incarnation. Humanity reproduces through the planting of sperm (that which gives form and shape) in the 'field' of the womb, which field supplies the matter of our human nature. Original sin is passed on through sexual intercourse, and thus, in order to be without sin, a conception is required which bypasses such intercourse. What is needed is to replace the sperm with a different force providing form and shape to the matter of humanity - this is supplied by the Holy Spirit. Thus the doctrine of the Virgin Birth explains and supports the doctrine of the incarnation, that Jesus Christ was fully human (all of his material was the same as our material) whilst still being wholly divine (a sinless image of the Father).
The trouble is, we don't understand conception in the same way any more. And to believe the same thing in a different context is no longer to believe the same thing. In other words, when weight is placed upon this doctrine the consequences that follow from it are not the same consequences that flowed hitherto. As I see it, a doctrine that once explained and supported the doctrine of the incarnation now accomplishes the precise opposite - which will be the subject of the next two posts.