That Auckland billboard showing Joseph and Mary in bed together - he looking disappointed, she wistful - produced a tawdry own goal against the church that displayed it.
St Matthew-in-the-City's intention was apparently to get people talking about the meaning of Christmas as God entering human experience in a dramatic new way.
Instead the caption - "Poor Joseph. God was a hard act to follow" - focused their imaginations on the sexual prowess of the parties involved.
That is a pity, because there are legitimate questions about the biological aspect of Jesus' conception, but that is emphatically not one of them.
The gospels of both Matthew and Luke tell of Jesus' birth to a virgin.
A previous article (ODT, 11.12.09) described how myth, midrash and the religious climate of the Mediterranean world in their day helped shape their accounts.
The question remains: was it physically possible for Mary to bear a child without the aid of a male's sperm?
In many church circles, such a question would be ruled out of bounds as disbelieving, distasteful or blasphemous: the virgin birth is a cornerstone of their religious faith.
They must not, however, presume to deny others the freedom to explore it in an open-ended way.
Obviously enough, Matthew and Luke wrote without benefit of modern knowledge of reproductive biology.
It was not till 1826 that a German-Estonian embryologist, Karl Ernst von Baer, established the startling fact that a woman actually contributed something of her own being to the process of conception, in the form of an ovum.
Till then, the received wisdom was that a man's seed was planted in a womb waiting to accept it, much as a poppy or petunia seed might be sown in a flower pot.
The woman's sole role was to bring the seed to fruition.
Baer's discovery poses a double problem for a virgin birth.
Not only is the male contribution necessary for reproduction, but if an ovum were somehow to develop spontaneously into an embryo, only a female baby could result.
It requires a Y chromosome from the male sperm to conceive a boy.
Literalists might argue that God can do anything, even change the laws of reproduction, or get an angel to supply a sperm substitute, or cause the fetus to mutate from female to male.
Some Catholic theologians have been so concerned to uphold Mary's virginity that they have argued not only was Jesus conceived miraculously, but so was Mary.
This was done to elevate Jesus and Mary above any taint of original sin (whereby Adam's sinful state is presumed to pass to every new baby through the sex act).
Such speculation is not persuasive to the modern mind.
If a literal virgin birth is no longer convincing, two other interpretations are open to us.
Neither need take anything away from the faith affirmation that Jesus is, in a unique and powerful sense, the human face of God.
First, Jesus could be the son of Joseph in the full sense of the word.
Against that is the New Testament record that he was not the cause of Mary's pregnancy: she was engaged to him, but they had not yet come together.
Being unmarried and pregnant to someone else was a serious matter, and Joseph would have been within his rights to repudiate her.
Instead, to avoid exposing her to shame - or worse, risk having her put to death by stoning - Joseph stood by her.
The other alternative is that the young Mary was violated or seduced by some unknown person.
Not surprisingly, many Christians are repelled by the very thought of this.
It flies in the face of centuries of regarding Mary as the quintessence of purity, far beyond anything that the Bible itself sets out.
From time to time in the gospels, however, there are hints that some of Jesus' contemporaries harboured doubts about his origins.
One effect of the virgin birth stories is to rebut such unsavoury rumours by giving Jesus an unimpeachable pedigree, through the Holy Spirit no less.
Perhaps it takes an era like our own, where the stigma of illegitimacy has been removed from children born out of wedlock, for Christians to be able to contemplate that a natural birth, not clearly accounted for, detracts nothing from the character, meaning and purpose of Jesus.
If his birth proved to be as scandalous to the upright as his death on a criminal's cross, but he still lived and died Godness, the wonder of his life would be the greater, not the less.
• Ian Harris is a journalist and commentator.
