Jeff and Sarah couldn't stop themselves, they were so turned on by their messy diapers, they excused themselves from Jeff's mom as soon as they could without arousing suspicions, and had an extended round of the hottest sex they could ever imagine.
It didn't matter that crap got smeared all over. It didn't matter that they smelled of urine and shit. All that mattered to them was how hot they were and how much they needed to get off. In a world where everyone wore diapers, they had created their ultimate fantasy - and something that was never meant to be.
Everything was changed. From the stock market to social practices, and everything in between. Diaper sales skyrocketed, as one might imagine. So did baby powder and cleaning supply sales.
Doctors started seeing a rash of... well... diaper rash cases. No matter how much baby powder and how much cleaning, people were never meant to wear diapers their whole lives, and so these changes had definite and far-reaching consequences far beyond Jeff and Sarah.
Scat become more and more common and practiced even main-stream. General health of the population took a dive at that point.
Eventually, people started living shorter lives because of the unhygenic new norm. And beyond that, as things got worse, people started dying young of all kinds of infections, to say nothing of the painful rashes, breathing problems and other health issues that started developing from this "new normal".
Jeff and Sarah just wanted to get off on a fetish. It makes one wonder, had they known it would spell the destruction of the entire human race within 30 years, would it have made a difference?
Or was getting off on their fantasy even more important to them than even the survival of the human species?