Book 3, Chapter 34
Guenever is hard to explain, because she is hot-tempered and jealous and possessive, but she is also generous, and faithful, and the men who were drawn to her were good as well. She was not afraid to take what she wanted, but she tried not to hurt others in the process. "Guenever's central tragedy was that she was childless." Book 3, Chapter 34, pg. 498 And she was made for children-she would have been a great mother.
So perhaps she took Lancelot as a lover because she had no children to focus her love and attention on. Now, she knows that even though he thinks he cannot come back to her after seeing the Grail, he will. She does not see this as a betrayal of God-she thinks they can have God together. Ultimately, while he thinks their affair is wrong, no matter how much he loves her, she does not believe it is.
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