The nineteenth century simultaneously discovered the Middle Ages, that "glorious age of chivalry," and decided that everything bad could be described as medieval. If there wasn't enough bad stuff, they made some up. Among these was the chastity belt.
The story, of course, was that when Crusaders left for the Holy Land they locked their wives into chastity belts to make sure that nothing untoward happened while they were gone. The difficulty is that there isn't the slightest evidence of anything of the kind.
The only thing that could even vaguely be interpreted as supporting the existence of chastity belts (and probably where the nineteenth century got it) was the statement by some twelfth-century monastic leaders that knights should "put off the belt of war" [i.e. stop going to war with a sword strapped to their hip] and "put on" chastity and obedience as monks. You notice they were urging men to live chaste, obedient lives as monks, not recommending a specific belt. You also note they were addressing men, not Crusaders' wives.
The first actual thing that could be interpreted as a chastity belt was a fifteenth-century drawing of an iron contraption, in which a writer complained that Florentine women inserted themselves. One wonders if in fact this writer was "getting no satisfaction" (to paraphrase the Rolling Stones). You also note that this was long after the Crusades were over.
By the nineteenth century, people were claiming to have found chastity belts in women's graves. One does of course wonder why a woman would be buried with such a thing. After all, it was far too late to worry about keeping her chaste. Some of these were put on display in museums but more recently quietly removed, because the contraptions appear to have been fakes, constructed in the nineteenth century.
Let's think about the image of chastity belts for a moment. The idea is some sort of plate that went between women's legs, to keep them safe from adulterous activities. For starters, it would be extremely uncomfortable. And of course the woman would have to remove it to use the garderobe, or to clean herself up every month. Otherwise, it would have been filthy in no time. If a woman could remove it any time she needed, what was to keep her from removing it when a handsome young squire caught her eye?
The real question is what made people dream up chastity belts in the first place, or why they still have a hold on the imagination. The Victorian era was fascinated with sex, which was supposed to be secret and forbidden and therefore twice as exciting as if it were out in the open. A chastity belt allowed people simultaneously to talk about "sex is bad" (and the romanticized Middle Ages supposedly knew this!) and to get all intrigued by things between women's legs.
It always makes people feel better to think that earlier people did weird things. It makes the weird things we do today seem almost normal.