This is a thought that just came to me and is only partially developed...
The Da Vinci Code's thesis is that Mary Magdalene had Jesus' child and that this child becomes a "royal bloodline" in France. The blood of kings, royal blood ('sang real').
Theologically, that doesn't make any sense to me. If Brown said that the descendents of Jesus became social workers, organizers of international charities, or religious leaders, that would make sense. But earthly rulers. Doesn't fit.
In some ways, that argument by Brown is more of an affront to Jesus than any claims about his having been married. Jesus made clear that His kingdom was "not of this world." If Mary was the first and true leader of the new Christian movement, as Brown alleges, I think she would have imparted a different approach to her offspring, one more consistent with the teachings of Jesus.