Do you think it was really used in medieval time?
Try and imagine what the result would be if any such device was actually used for long periods of time. And let's not posit a jealous husband a few years away on Crusade, but say just 6 months away.
Now, hygiene was maybe never a particularly strong suit during medieval times, but try and imagine the residue of urine and feces that would accumulate in such a device after even a short period of time. In those times people people died of all manner of bad hygiene related diseases, suppurating sores, infections, rotten teeth and whatnot. Wearing one of these things would just increase the likelihood of becoming infected, as it would not be possible to clean oneself even perfunctorily. And just imagine what would happen to the flesh around the inner thighs with metal edges rubbing against it for months on end. More infections, unhealed cuts and abrasions, maybe even gangrene?
Chastity devices are probably more the result of an erotic imagination than anything else. Play-toys for enactments or else show pieces for traveling exhibits of torture devices and other odd and terrifying machines. From the 18th century and on, it became very popular for overly affluent individuals to collect odd and rare devices - cabinets of rarities such collections were often called. They would also be used in traveling shows, at fairs, sort of like what used to be 20th century circus freak shows.
A chastity device, along with some torture devices, thumbscrews, a Nuremburg Maiden, a rack or guillotine and such are always great attractions. People love to see such stuff, just like they love to watch horror movies in our present times.
Most of the old chastity devices still in existence were made long after the Middle Ages, probably for the above reasons, not to actually be put to use.
Such traveling exhibits of torture devices (along with a chastity belt in iron) are still with us nowadays, though less frequently than in previous centuries. I've seen the same one in two different countries and while it's fun to visit, the disappointing part is that all the items on show are modern copies.