According to Arthurian History
The idea that Guinevere and Sir Lancelot committed adultery with one another is found only in the legends of Arthur and the knights. Geoffrey of Monmouth refers to Queen Guinevere as Ganhumara, and the Triads imply that King Arthur had three queens, all of them named Gwenhwyfar, the Welsh version of Guinevere. According to Welsh tradition, there were two different outcomes to Gwenhwyfar’s abduction by Melwas: the first has King Arthur coming to her rescue, killing Melwas and rescuing her. The second version has a character named Gildas arriving to solve the dispute and ultimately ransoming Guinevere. Over the time, the legends would change Guinevere’s rescuer to Sir Lancelot and romanticize the rescue, furthering the divide between King Arthur and his first knight: Sir Lancelot. The rescue would also add to the love triangle between Arthur, Guinevere, and Lancelot.
But as we know and have addressed in other posts, Sir Lancelot is almost assuredly the creation of French Poet, Chretien de Troyes, and therefore can’t really be any part of a true historical investigation. According to Geoffrey of Monmouth though, who calls Sir Mordred, Modred, Guinevere assists Modred of her own accord, in helping him usurp the throne from Arthur and ultimately betray him. Later on, writers would exonerate Queen Guinevere, and say she was forced against her will to help Modred. But Geoffrey writes that she not only assisted Modred, but would later become his queen and rule next to him. Like other legends, when King Arthur returned to battle his nephew Modred, Guinevere would then retreat to an abbey and live out her days as a nun in solitude.
According to Arthurian Legend
French Poet, Chretien de Troyes is really the one to give us the first version of the Sir Lancelot and Queen Guinevere adultery story. de Troyes said that Lancelot and Guinevere loved each other deeply and gave themselves to each other heart and soul. They were shameful, yet unashamed of their love for each other at the same time. Other writers would take de Troyes’ story and run with it, the first being Sir Thomas Malory in Le Morte d’Arthur. Malory approached it from the angle that King Arthur almost willingly turned a blind eye to the love between Guinevere and Lancelot, and in doing so began to lose the trust and confidence of his knights, who thought if Arthur couldn’t manage his wife and closest friends then how could he rule a kingdom?
According to Malory, King Arthur would ultimately condemn the Queen sentencing her to death by burning at the stake. The rest of the story is well known, with Sir Lancelot riding to her rescue, accidentally killing Sir Gareth, and escaping with his lover. Arthur and Lancelot would fight over time, and ultimately Guinevere would end up in that nunnery the same way that Geoffrey writes that she did.
In the writings of Alfred Lord Tennyson, Lancelot’s sin is what ultimately brings down the court and keeps him from having a full vision of the Holy Grail. Modern authors would take a slightly different approach to the story, writing that the relationship between Guinevere and Lancelot was inevitable and almost understandable since Guinevere never really loved King Arthur and was almost forced into a marriage with him.