Well, to Lancelot's defence, he was seriously tricked. He thoguht he'd be spending a night with Gwen/Guinevere, while it was, in fact, Princess Elena, put under a transformation spell by the Dame Brisen (all canon stufff, BTW). She was supposed to have a baby with Lancelot, but Lance was so fixed on Gwen that he wouldn't knock her off otherwise, so it isn't really his fault.