Brown obviously looked at stuff like The Holy Blood and the Holy Grail and thought "There's a yarn in there!". He'd already done the same with the Illuminati and went on to do it with the Masons.

As to the thiing about Jesus and relationships, that's Christianitys' own fault! Paul the Hellenised Jew and the other Greeks who turned Jesus' apocalyptic cult into a Mediterranean mystery religion also added a generous dollop of Stoicism into the mix. The Stoics were ascetics and Jesus believed that since the end of the world was due in a couple of years, there was no point making babies. The result was a religion that was very down on sex. But most people like sex. so the idea that the founder of the religion that was telling them to keep it in their pants was actually sexually active makes for a nice, big 'gotcha' moment!

Was Jesus sexually active? How could it possibly matter? If he was an ordinary 30-year-old carpenter from Nazareth, then he was probably married with kids. If he was an actual Rabbi, then he would have to be - it was required. If he was an Essene, then he was probably celibate.

Are any of Jesus' descendants around today? If he was married and a father, then probably. So what? Genetic surveys indicate that we are hip-deep in descendants of Ghengis Khan, but nobody thinks we should start up a horde because of them! (Well, some people might...)

As to Brown believing what he wrote - well saying so was probably a Hell of a way to sell more books, right?