A controversial billboard put up by an Anglican Church in New Zealand has prompted some debate (as was intended):
According to an article about it, the sign was defaced within hours of being put up. "Church vicar Archdeacon Glynn Cardy said the sign was intended to challenge stereotypes about the way Jesus was conceived and get people talking about the Christmas story, but not everyone has taken it that way. Catholics, in particular, were upset."
At issue is the question of whether Mary was perpetually a virgin or whether she had relations with Joseph after the birth of Christ. Another, even more controversial issue is whether Joseph was the biological father of Jesus.
Now, the question of whether Mary was perpetually a virgin (even after the birth of Jesus) fascinates me. I think the arguments in favour of perpetual virginity are kind of weak and derivative. Scripture would seem to suggest that Mary and Joseph did have "relations," after the birth of Christ: "When Joseph awoke from sleep, he did as the angel of the Lord commanded him; he took her as his wife, but had no marital relations with her until she had borne a son; and he named him Jesus" (Matthew 1:24-25, emphasis mine).
Then there are references to the brothers of Jesus. Multiple times in scripture there are references to the brothers of Jesus. Consider Matthew 13:55, Mark 6:3, Galatians 1:19, and 1 Corinthians 9:5. Even the ancient historian Josephus in Jewish Antiquities (20.9.1) referred to "[James] the brother of Jesus who is called Christ." Josephus does not use the term for "half-brother" or "step-brother" as he does when describing other such relationships, so that strongly implies that at least Josephus thought James was the Lord's full-brother.
Although the Magisterium (the Roman Catholic teaching authority) may argue that these could be step-brothers by a previous marriage of Joseph, it's hard to square that with the fact Jesus' name always appears first in the lists (suggesting he was the oldest). Also, why is there no mention of the previous children of Joseph in any of the nativity narratives? Did these kids go with Mary and Joseph to Bethlehem or what? Also, consider the fact that in Judaism a man can only have one first born son. So how do we make sense of Luke 2:22-23 referring to Jesus as Joseph's first born son if James was actually older?
A very old (circa 150CE) document called the Protevangelium of James makes the argument that Mary was betrothed to an older relative in order to preserve her virginity, and that sex with Joseph would have been incest. But this document was never considered canonical as scripture, anyway. Certainly it was not written by James (the author was unfamiliar with certain aspects of Jewish culture and nor does the writing style of the Greek make earlier authorship unlikely). However, this non-canonical Gospel does demonstrate that devotion to Mary and the tradition of her Perpetual Virginity are quite old.
Indeed, the real arguments for the perpetual virginity of Mary are not based on scripture but upon theology (or, more accurately, "Mariology"). In other words, it's based on a theologically nuanced view of the role of Mary in salvation history and the implications that has for our understanding of her, that we begin to make an argument for her perpetual virginity. I'm not going to rehearse that argument here, but suffice it to say that many fine Christians find it persuasive. So much so, that the doctrine of perpetual virginity is considered "essential" to the Catholic Faith. That is, it is considered beyond question.
For me, I think that Joseph and Mary had normal marital relations after Jesus was born. But I don't feel it's very important to convince people of this. If Mary's continued virginity is important to your faith, go for it. God knows there are going to be many things like this that people of good faith will disagree about when they come together to worship and adore the Son of God and his Blessed Mother. As for me, the notion that Mary and Joseph had kids after Jesus enhances, rather than diminishes, my devotion to the BVM and her Son.