Aquinas' argument leaves Jesus' humanity intact - he is of the same stuff as us. Yet if we take a different understanding of conception, ie one which involves DNA from two parents being fused to create a new individual, where does that leave Christ's humanity? One might easily say 'God created half of Jesus' DNA' - but what are the implications of accepting that? One is that Jesus possesses 'special DNA', he isn't part of the common flow of DNA mixing - and therefore he is special and unique. That might not sound surprising, but the particular form of the uniqueness is no longer what it was for someone like Aquinas.
Consider, for Aquinas, the special and unique thing about Christ was the ordering of his human material, in such a way that it was without sin. In other words the distinction between Jesus and the rest of humanity was between the one who was sinless and the ones who are embedded in sin. This is a spiritual distinction, a moral distinction, a distinction which - at least in principle - is a consequence of voluntary actions, either of Adam and Eve originally, or of ourselves as we take part in the normal human activities of a sinful world. In other words there is no absolute distinction between Jesus and the rest of humanity - indeed, he shows us what being human really is, what being human was intended to be and, if we accept this, then through grace we might be able to share in what he has accomplished. Hence "to all who received him, to those who believed in his name he gave the right to become children of God - chidren born not of natural descent, nor of human decision or of a husband's will, but born of God".
None of this is possible if we understand conception differently. For if Christ is the product of 'special DNA' then the distinction between his humanity and ours becomes absolute. No longer is it something over which, even in principle, any human being can have any choice. A barrier has been established between the humanity of Christ and the humanity of the remainder of the human race. No longer is the barrier one to do with sin and choice and responsibility; now the barrier is one of biology and DNA. Jesus is different from us, not primarily because he was sinless (though that remains the case) but because his sinlessness proceeds from his biological distinction - in both sense of that word.
Jesus' humanity is no longer the same as our humanity - and what that actually means is that Jesus is no longer human. He seems to be human; he acts on earth as if he was human; but it is a charade. Jesus is Superman, with an in-built genetic advantage. As such, he cannot be our saviour. For what he has not assumed, he has not healed. He has not assumed our mortal flesh, consequently our mortal flesh is not redeemed, is not raised up, is not capable of bearing divinity. I'll say more about that in the next post.
For this one, what I would end with is a quotation from "Thirty years of honesty", a prior version of which I read at University, and this quotation has always resonated with me, as it sums up all that seems to evangelistically unhelpful about this doctrine (my emphasis in the text). A 'woman's voice from a vicarage' wrote to John Robinson saying:
The orthodox teaching has always maddened me; so many other humans have sacrificed themselves for us, endured more sustained and prolonged torture, without the comfort of being 'the favourite son'... Any of us knows the impossibility of the struggle; what you have helped to remove is my constant annoyance that Christ always had an unfair advantage.