In the Prologue to the "Wife of Bath's Tale" the Wife relates little antecdotes about her life and her many marriages in particular. A running theme of her view of life and marriage is that the best and happiest marriages are those where the woman has the power and control. In her last marriage her husband tried to take the "reigns" and lectured her about a woman's place from a book. The Wife had had enough so she grabbed the book and they ended up in quite a tussle over it; the Wife was so badly hurt the husband thought her dead. When she came to, he was SO relieved he immediately gave up his old attitudes, and as she says, "He gave the bridle over to my hand, (and) gave me the government of house and land." She later states that she "mastered him," and explains that their life together was much better after that point.
All of this personal background gives some context to the kind of story that the Wife of Bath would tell. In her tale, the arrogant knight must be brought down under the control of the woman in order to be completely happy. In order to get the answer to the queen's question and save his life, he must make the fateful choice: a wife who is ugly and faithful, or beautiful, but deceitful. Instead of making a decision at all, he leaves the decision up to her -- giving her complete control -- and he is rewarded for it by the old women turning into the best of both worlds -- beautiful and faithful. The Wife of Bath uses this fictional fairytale to reinforce the theme of her own life. Men who give control over the women in their lives live a much happier and peaceful life.