Theologians do not usually think of the doctrine of the virgin birth as being an act that involves a sexual commitment on anyone's part. In two of the Gospels, all Mary has to do is give her consent: the Magnificat - a resounding "yes" to God. And the finer details of how Jesus came to be flesh are lost to the more interesting stories about the nasty Herod, enthusiastic shepherds, wise men and even more angels. In classic Christian iconography, Mary is portrayed as conceiving Jesus through her ear - she heard the word, and so Christ was formed. Aural sex has never been more productive.
Traditional Christian doctrine affirms, through the Chalcedonian Definition (451 AD), that Jesus is both "truly God and truly man . . . two natures, without confusion, change or division, preserved and combined in one person". This definition, like so much early Christian doctrine, was an attempt to express the mature mind of the Church, make sense of the mystery of revelation and salvation - define, but not confine God.
Yet there are real problems with imagining what it is like to be incarnate through a non-sexual act. Alas, there is little biological evidence to go on: no bones or tissues to analyse. Depending on your point of view, it is either resurrected to heaven, or buried in a long-forgotten grave. The genetic identity of Jesus's father cannot be known. There might once have been some hope. Several medieval cathedrals in Europe competed with each other over the claim as to who had the genuine foreskin of Jesus, following his circumcision (Luke 2:21). There was even devotion to this holy relic, and some mystics had experiences of and saw visions of its power. But nothing has survived to this century.
Theologically, this presents something of a challenge to the idea that Jesus was fully human and fully divine. How can you be a human person without having a human father? Surely flesh of our flesh, bone of our bone, is also DNA of our DNA? If Jesus does not have these human qualities, then how is salvation to be thought of? Gregory of Nazianzus (360 AD) said that "what was not assumed was not redeemed", arguing that if Jesus wasn't really human, we were not really saved.
Some modern theologians have attempted to side-step the issue. Even the most radical can usually assent to a statement like "God was in Christ". The problem is this: how much of him? Are we talking about a person uniquely imbued with divinity, infused with God as it were, a bit like a spice in a meal? Or another person who was a pre-existent being in the perfect communion of the Trinity, but who once learned carpentry skills?
No theological musings help the biological and genetic questions posed. But then, why should they? The early church didn't know anything about genes. It sought through its creeds, councils and controversies to safeguard unity, promote peace and snuff out heresy; and then to try and capture the mystery of the salvation wrought through the incarnation, in forms of words that did justice to a God that was sufficiently committed to humanity to become one of us. To live as a human, love like one, die as one, and yet not let that be the end of the story.
Heresy often seems to make more sense than orthodoxy - it is usually a reductive, comprehensible account of a mystery. Orthodoxy knows it cannot afford too much rationalism when it comes to the same, which is why it is so difficult to believe. But the struggle to think is always worth it, for without that, there can be no real living of the mystery. To follow Christ is to be caught up in ambiguity, to lose your certainty, to watch the light, but often in the dark.
So, there is no theological solution to the biological questions. We can't really know what went on between God, Mary, Joseph, and the sheets. All there can be is poetry, faith, hope and love. And the inadequate theological formulas that try to configure the amazing reality of the Christmas story - that he is, somehow, "Emmanuel". God, with us.