What are the implications for saying that the Virgin Birth must be true, even when the content of that belief has changed so much? One of the most malign, it seems to me, is the intensification of the gnostic tendency in Christian thought. That is, in the history of Christianity there is a strong streak of hostility to the body. I do not see this as a necessary feature of Christian belief, in fact I think the opposite is true - that is, the entire logic of the incarnation is that human flesh is capable of bearing divinity, that the image of God can be revealed through human skin. Yet there have always been more or less Platonic elements which have fostered the idea that the flesh gets in the way, that read Paul's language about the flesh in a biological (rather than symbolic) way and so treat the flesh, and the desires of the flesh, as inherently sinful.
Now the Aquinas form of understanding the Virgin Birth, whilst it has a negative view of human sexuality (as that which passes on the 'infection' of original sin) need not have a negative view of human flesh as such. For it is still the human flesh that Jesus took on and sanctified, and as such we who share that human flesh are also able to be sanctified.
This is not possible if Jesus is biologically distinct from us. The whole process of fleshly existence is suspect, for our flesh is not capable of bearing the incarnation - special DNA is required. If the incarnation is not possible through the normal means of human reproduction then that means that God's purposes are inhibited by normal human sexuality. It means that human sexuality is downgraded; it is inherently less worthy of respect, it is less capable of redemption and transcendence. This has led to untold harm and evil in human life, most especially our Western culture. I can't help but believe that a faith which told of God being present through and from the normal processes of human sexuality would have a much more healed and healing understanding of sex than ours does - our church and our wider society.
For what is at stake in saying that God cannot incarnate through sexual union? What is being preserved? I can understand someone wanting to uphold the authority of Scripture hanging on to the doctrine, yet if they also see these malign consequences then the doctrine must surely lose its capacity to 'bear weight'. For as soon as it does start to bear weight, then it seems that these malign consequences follow. Instead of being a faith that celebrates the possibilities of our creatureliness, that exalts in the possibility that God is seen in flesh, we have instead a repressed and repressing gnosticism, that cannot bear that flesh would bear the divine - and this hatred of the body is then reinforced and promulgated: a faith of flagellation rather than a faith of carnival.
And this too has evangelistic consequences.