Historical opinions aren't formed on what is DISproven - it's simply completely unreasonable to do such a thing. What
is reasonable, though, is to point out that in the most historically reliable documents there is absolutely no indication whatsoever that such a relationship between Jesus and Mary Magdalene might have existed, and the absence of such a relationship is not an unusual thing by any means whatsoever.
Likewise, the reason the assertion that Mary Magdalene was pregnant at the crucifixion (let alone with Jesus' child) is laughable is because there is
absolutely no reliable historical evidence to the point. The point is not that it's categorically false, it's that there is no good reason to hold a belief that it might be true. Short of doing all the historical research yourself and becoming one of the leading scholars on early Christianity and the historical Jesus like Prof. Ehrman has done, there's not much reason at all to place doubt on his opinion and research in favor of that of Dan Brown, an author of already otherwise historically inaccurate fictional books.
BTW, Oxford University Press has
a page with some writing by Prof. Ehrman regarding Mary Magdalene (I believe it is an exceprt from his book,
Peter, Paul, and Mary Magdalene: The Followers of Jesus in History and Legend). NPR's "Fresh Air" also has a very good ~40 minute
interview with Prof. Ehrman available online in which he speaks quite a bit about the issue as well.