It may be somewhat misleading to call the Wife of Bath's long speech about sex, marriage, and her storied history a "Prologue." As the Friar says at the end of it: "This is a long preamble of a tale!" (831). In fact the Wife's Prologue has as much to do with her as with her tale, and is twice the length of the tale itself. The Wife seems to have taken her opportunity to speak as a perfect occasion to defend her way of life and speak about something in which she deems herself an expert – marriage.
A more accurate title for this part of The Canterbury Tales, then, might be "The Wife of Bath's Musings on Marriage." Even that, though, doesn't really capture the complexities of the Wife's speech, the way it shifts from a defense of marriage to a warning against it, and spends as much time on the Wife's own history as it does on a more abstract argumentative track. Maybe it's best just to stick with "The Wife of Bath's Prologue" as the title for what we will probably never be able to categorize any better than that.