With regards to the statement of Archdeacon Cardy that the intent of the billboard was also to disabuse people of the notion that God is a male, it may succeed less in this regard. In fact, the poster would seem to suggest that Joseph can't live up to the sexual prowess of a far more potent male, God, "a hard act to follow."
It is in turning to the actual Scriptural account that one may more effectively banish the patriarchal notion of a male God. In the Lucan story of the Annunciation to Mary, when she questions how her pregnancy is to take place, the angel Gabriel replies, "The Holy Spirit will come upon you. ..." (Luke 1:35). The Greek (the language in which the text is written) word for "spirit" is "pneuma," neuter in gender, while in Aramaic, the language spoken by Jesus, the word is feminine. Thus, the Scripture seemingly offers a more gender-free understanding of how this pregnancy is initiated than the controversial billboard.
Whatever your take on the New Zealand billboard, it has no doubt stirred theological conversation about deeply held beliefs about the Christmas mystery. If the conversations lead us deeper into that mystery, then perhaps the goals of the billboard's sponsors have indeed been met. However, enabling those conversations to continue in productive and perhaps less salacious ways may be the real hard act to follow.