Reversed situation.

In this world due to the **** being invented and it's effect transmitted to the next generations the girls become addicted to cum the first time a boy cums in their pussy.

A few options were studied and hastily rejected, finally it was decided to fit each girl in a chastity belt at the onset of puberty, other experiments having shown that teenage girls couldn't be trusted to not looking for a dick to stuff in their pussy.

They can masturbate once a week by asking their parents to activate the vibrating device embedded just over their clitoris but the belt would only come off when they are given away to a boy that would be in charge of their sexlife for the rest of their existence. (the last generation of belts are hydrofuge to keep clean and grow with the person wearing it, leaving no reason out of medical emergency to remove it.)

You are standing there in the street with the knowledge of this world pouring into your brain, when you feel around your neck the weight of a small chain and the cold contact of a small key on your skin under your shirt.