I posted this a while ago:
A young man lives in a magical society of some kind or another. He is placed into chastity as a punishment for something he did to piss off the higher-ups of said society. They put his junk in an intricate and obviously enchanted chastity cage/belt and send him on his way.
Nature takes its course and he gets hornier and hornier over the span of a few days (naturally, not magically). He begs his warden/keyholder to let him cum, agrees to do ANYTHING the warden says as long as he gets to cum. So the warden lets him out... but the chastity cage has some kind of spell on it that turns the previous wearer female upon being taken off. So now our hero is female, but is still horny, and he allows the warden to fuck the shit out of him. He cums, enjoys the sensation of the female orgasm and/or is repulsed by the sex act itself, everything is right in the world... but he doesn't turn male again. Only after the warden re-applies the chastity device does our hero get his dick and his male body back.
So now he's stuck in a life where the really important parts of his life (job, family, friends, etc) take place while he's male and locked up, but he has a high libido and can't cum unless he's female. Meanwhile, the keyholder keeps ratcheting up the things our hero has to do in order to be let out of the cage, which of course leads to our hero doing all kinds of insane/sexy/pervy shit in order to have the opportunity to cum.
Near the end of the story, he decides to just stay female all the time because it's less of a headache.