That would depend on which account you are reading. In the Prose Lancelot an account is summarized that Arthur, in his early days, heard about the glory of King Leodagan the King of Carmelide in the...
King Arthur had a wife named Guinevere, who left him for Lancelot, a knight. There are some spelling variations in different legends. Technically, she doesn't "leave" Arthur for Lancelot. The affair...