It has been a few years since The Da Vinci Code was a literary sensation and created something of a stir in Christian circles. If you recall, the novel asserted that contrary to accepted church history, Jesus and Mary Magdalene actually married and had a child. The novel's protagonist, Robert Langdon, discovers this secret and learns that the Holy Grail is not the cup that Christ used in the Last Supper (as it is portrayed in the movie, Indiana Jones and the Last Crusade) but rather it's Jesus' bloodline that began in the womb of Mary Magdalene and has continued down to the present day, protected by a secret order (the Prior of Sion) against the Roman Catholic Church, which will stop at nothing (including murder) to suppress this truth.
Empirical evidence for this version of the legend of the Holy Grail is found in two of the Gnostic Gospels, in particular, the Gospel of Philip and the Gospel of Mary Magdalene, both of which state that Jesus kissed Mary (Magdalene). The Gnostic Gospels are some of the most famous "gospels" that are not included in the Christian New Testament, and a theme that runs throughout The Da Vinci Code is that the Roman Catholic Church suppressed the Gnostic Gospels because they celebrated human sexuality while the Church (and the Gospels it included in the New Testament) did not.
Such an assertion, however, is patently false. Wishful thinking aside, the Gnostic Gospels actually hold a negative view of sexuality. They see the material world as evil and believe that the path to salvation lies in "cultivating the spark of gnosis (from the Greek word for "knowledge") implanted in each person" (Murphy, p. 70)
"In these gospels, you won't find any bodies healed, any meals with sinners, any enjoyment of the earth, and any teachings about marriage. In fact, the only intercourse that happens in these texts is the union of of human reason with divine reason. These gospels were labeled heretical because the mainstream Church had a more positive view of human bodies, sexuality, and the material world in general. They (i.e., the mainstream Church) valued the human and historical Jesus" (Murphy, p. 71).
Thus, the kiss between Jesus and Mary is probably better understood symbolically, as the divine revelation that passes from Jesus' lips (Murphy, p. 72).
Don't make the mistake of dismissing Catherine as a reactionary conservative simply because she teaches at a Roman Catholic university. Trust me, she isn't. I know and have talked with her about the quest for the historical Jesus. She teaches an undergraduate class on the topic at Santa Clara, much of which has been captured in her book, The Historical Jesus for Dummies. (Don't let the fact that it is part of the "Dummies" series fool you. It is an excellent summary of current research and is far more accessible than the books by Crossan, Robinson and Meier.)
Another book worth exploring if you're interested in some of the historical and theological problems of The Da Vinci Code is the book by the agnostic/atheist New Testament scholar Bart Ehrman (he lost his faith while getting his PhD in New Testament), Truth and Fiction in the Da Vinci Code.