'You increase it by doing it often, it's slow at first.' Just spam it like 10 times in sex mode.
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It's a bug, it occurs when you wear a chastity cage that your character can't yet handle wearing, it will take it off after fast forwarding a day. However, if you wear that kinda cage and give the key to someone, the chastity will be removed, but the keyholding person will still have the key. You can't fix it, unfortunately. I suggest you wait for the update so you can enjoy the game rather than focusing on bugs. Thanks.