The Myth of the Downfall of Camelot

There is a myth concerning King Arthur and Camelot that has been perpetuated by the Roman Church. Many of you have seen movies and read books concerning Arthur and the Knights of the Round Table and how the king and the land suffered as a result of an indiscretion. But what was the indiscretion? Legend informs us that Guinevere had an affair with Lancelot and because of this infidelity Camelot was plunged into darkness (metaphorically) and Arthur lost his will to govern. But was this indiscretion really the cause of the downfall of Camelot?