Today, with the advent of genetics, Christian thinkers now know that Mary must have contributed the female egg that made Jesus into a man. [Jesus, being a male, could not have been her clone, otherwise he would be a woman, and if cloned purely from Mary’s genes would nullify the claim that he was God’s son, too]. This presents a problem for we know that in order to be a human being in the first place, it takes a male sperm.
The ancients commonly believed that the woman contributes nothing to the physical being of the baby to be born. The mother was nothing but a receptacle for the male sperm, which grew to become a child. So the ancient and medieval church believed that Jesus’ humanity was a new creation, and therefore sinless. Modern genetics have forced Christians today to take a new view of the virgin birth. Can they adequately explain how Jesus is a human being, since a human being is not conceived until a human male sperm penetrates a human female egg. Until that happens you do not have the complete chromosomal structure required to have a human being in the first place.
Of course, Christians can always say God can do anything, even create a human being. But then how is Jesus a descendant of Joseph? Neither Luke's nor Matthew's geneologies genetically link Jesus with Abraham or Adam. He's not the "real" father of Jesus.
And as far as punting to the "God can do anything" defense," Christians only use that defense when they need to use it, for you don't hear them saying this when it comes to the problem of human suffering in the world. You don't hear them arguing that God could eliminate evil. "He has his reasons," they say.
It's just that at an ancient time when people thought virgin births were genetically possible, people claimed they happened, that's all. Does anyone really think they happen today? How hard would it be to convince someone in today's scientific world that a virgin had a baby, which was sired by God, and yet geneologically linked to a great great great...grandfather?
There are too many improbabilities here. Way too many.