I am uncertain as to why any who are not catholic would be upset over this movie, really. The movie is flat out wrong in many ways, but it does touch on some theories that have been around for a long time. The RCC considers the theories about Mary Magdalene to be heresy, but since when do protestants care what the catholic church rules is heresy? Protestantism is heresy.
But as far as Christian doctine and the Bible are concerned, I am unsure how the theory about Christs descendants, even if true, would change anything. How would it invalidate anything that the Bible teaches us? Though it has some pronounced inaccuracies (it IS fiction, remember), I do not recall reading anything in the story that was contradictory to the Bible or undermines Christian beliefs. If Christ DID marry, and the Bible did not tell us about it.. Big Deal. Apparently God did not think we needed to know that.. who are you to second guess him?
As I've said in other posts, I have not read the book, but I had assumed that in the DVC:
1. Christ doesn't go to the cross and die.
2. The body of Christ is not ressurected on the 3rd day.
3. Christ does not ascent to be at the right hand of God the father.
All of which are standard Christian theology, hence my use of the term "alternative."
It undermines the Bible itself.
Man those trees are in the forest's way for you.