Sabrina the Teenage Witch

Thousands of years ago, Witches discovered that constant sexual activity kept them young, healthy and magically powerful, so they enacted the following rules:

Rule 69.a: All Witches, upon turning 18, will be Free to Use for everyone around them aged 18 and older.

However after this Rule was enacted, Witches discovered that they began exuding pheromones which encouraged people around them to Use and be Used by. So the following was enacted.

Rule 69.b: Witches who live with under 18 year olds must take magical suppressants, in order to prevent under-age exposure to sexual activity.

To make things easier for those who live in the Mortal Realm, Witches encouraged Mortals to embrace Free Use as well. This has resulted in the following:

1: Senior Year starts the day one turns 18 and ends at 21. This is to ensure complete separation of under and over 18's. Thanks to Magic no one questions it.

2: Everyone under the age of 18 - be they witch or mortal - cannot see anything sexual.

But with this came another Rule.

Rule 69.c: While Mortals are naturally oblivious to Free-Use activity before turning 18, Witches are not. To help with this, all Witches under 18 are to be given Oblivious Potions to prevent exposure to Free Use before 18.