On a perfect day, a meteor skimmed past planet earth. Nobody was worried thanks to the correct calculations by NASA, and it was also an event that caught the attention of many people interested in astronomy, who observed the passage of the celestial body. However, something overlooked about the meteor had a huge impact on humanity, something that only showed its first signs years later.
The meteor emitted a strange radiation, which had been considered harmless, mainly because there was no impact with the Earth, but it was still enough to affect children from 1 year old and under, including those who were still forming in the womb of their mothers. These kids all grew up normally without showing any signs of anything weird about them, but as they reached maturity and their bodies produced more hormones, they started turning into sex monsters every time they got horny, changing back to normal when satisfied. The only effect on older people was to alter their genes so that even if none of them developed the mutations, all of their offspring would be were sex monsters.
When the first ones transformed there was chaos, taking some time to discover the condition and not knowing what caused it yet. At first teenagers were feared by others who hadn't turned once yet, at least until studies and exams confirmed that it was just that it would happen to everyone and it was just a matter of time.
This caused a change in many rules and customs, the main ones being that as soon as a person transformed they would have priority in relieving themselves to return to normal. As transformations were always unpredictable, the sewing method of clothes was reinvented, creating pieces of clothing that disassembled instead of tearing when they transformed, and could be easily reassembled and replaced after they change back to normal.