|Rossetti's 'Lancelot in the Queen's Chamber'|
Under chivalric law, a man has the right to defend his honor in Trial by Combat, meaning that the fellow who dares besmirch a knight's honor must face him, blade to blade, to prove the truth of the accusation. The same tradition carried over into the modern era, with gentlemen fighting duels solely for the name of their honor--and a scoundrel who won was the equal of a truly honorable man, except the scoundrel didn't actually have to be a good person in the meantime.
It was especially problematic for any knight who took umbrage at Guinevere's infidelity, since to accuse her was to invite a crossing of swords with the most formidable warrior of the Round Table. And indeed, every time such an accusation is leveled against Lancelot, he merely raises sword or lance and does away with it. The idea behind this method of justice was that a good and righteous God would not allow a just man to be defeated by a blackguard. Of course, the fact that the disingenuous Lancelot did win, over and over, just goes to show that the Arthurian balladeers of a thousand years ago had a more developed sense of irony and realism than many modern fantasy authors.