I went to a symosium on DaVinci Code at the local Barnes & Nobles about 2 years ago. They had experts from Rutgers on the panel - literature, history, theology, etc.
They all said the book as good entertainment. They all confirmed that the theory of a bloodline of Christ is not impossible. They reminded everyone that there were at least 12 apostles, yet only 4 are represented in the St James Bible. They mentioned that the Church of Africa is based upon the gospels of Peter - note, not Matthew, Mark, Luke or John.
They did note that not all the minute details were correct. They also mentioned that Holy Blood, Holy Grail is someone's thesis, but it is not considered good research because it only sets out to prove the theory and does not attempt to disprove it. So, there is a possiblity of some truth in the theory.
There's also no denying the Churches of Mary Magdalene in southern France.
The only thing that Dan Brown did was introduce the theory to a broader audience. The theory has been around since long before Holy Blood, holy Grail was published.
Personally, I'm not sure what all the hoo-haa is about, but I'm not religious. Based upon the little research I've done, the teachings of the gospels are the same whether Jesus was married or not. Isn't it more important to be a good person than to prevent people from learning more and forming their own opinions?