The blasphemy of the story isn't that he married or had children, but that the crucifixion was a hoax (or at least that Jesus never really died on the cross), and that he and Magdelene moved out of the region after they faked Jesus' death on the cross.
No crucifixion = no resurrection = no salvation for man through the blood of the lamb. That's what's really wrong with the story, and the one that keeps being missed in the press. Further, the novel suggests that the "real" teachings of Jesus are absent from the Gospels as part of a conspiracy of the founders of the Church, so the message you have is "wrong".