Guenevere stands accused of committing adultery, which was a pretty major sin in the eyes of Arthurian knights like Gauwaine. A woman's sexual purity was seen as one of the most important aspects of her character, so how could Guenevere allow even a hint of wrongdoing to touch her reputation? She admits to kissing Launcelot, but did it go any farther than that?
Questions About Sin
Who gets to define "sin" in this poem? What is his or her authority?
How does Guenevere redefine "sin"? Does her redefinition seem warranted in the context of the poem?
By whose definition has Guenevere "sinned"? Is there a way she could have acted that would not have been a "sin" by anyone's definition, including her own?
Chew on This
In "The Defence of Guenevere," Sir Gauwaine defines "sin" as any deviation from social convention, while Guenevere herself seems to define it as being unfaithful to one's own feelings.
In "The Defence of Guenevere," most characters would probably agree that the greatest sin is deception, both of oneself and of others.