I actually plan on creating a thread regarding this general idea, but I will respond here more shortly.
The Da Vinci Code, in general, is bull. It is an enjoyable (at least for me) read, but it is best to just understand it as a novel, and leave it at that.
Dan Brown's main sources were works that were not very credible. His largest used source was Holy Blood, Holy Grail, which was using documents that were known to be either forgeries, fakes, or simply incorrect (the authors seem not to have realized this, but their main source had even claim that he was making it all up).
Now, there were myths that had circulated about most of the individuals in the Gospels. There is one interesting one that speaks of Joseph of Arimathea (who, according to this myth, was the uncle of Jesus), took Jesus as a young child to Britain. But most of these myths are very late, and partly had to do with a need to connect with these figures, as well as for tourism.
So really, there is no actual evidence regarding what happened to Mary after the Gospel accounts.
Now, for Jesus being married, this is a possibility, that really isn't too far out there. Mary would be the most likely candidate, but it is something that lacks real evidence.
There are certain suggestions within the Gospels that could possibly allude to Jesus being married. Also, it would have made sense. But at the current point, a definite answer really can't be given.