Ben had always had the idea that it'd be interesting if everyone in the world swapped sex. He'd imagined writing a story about if every man in the world was a woman and every woman was a man, so he figured he could use this programme to get some ideas out there even if it didn't work. He typed the command, 'every human switches sex' and hit Enter.
A moment later, a weird sensation came over him, and he realized this thing worked.
The weird sensation also swept over the 7.9 billion other humans on the planet, and the Master PC brought everything to a standstill as it rapidly inverted every single person's sex.
Half the world changed one way, and half changed the other. Half the world changed from male to female- their penises shrank away, their testicles pulled up inside them, their scrotums unsealed and formed vaginas, their bodies shrank and became curvier all over, much of their body hair receded as their skin became smoother, and mammaries developed in their chests, with pubescent and older ones forming breasts.
The other half of the world changed from female to male- their vaginas sealed up, their clitorises lengthened and expanded into penises, their ovaries pushed out into a sac between their legs, their bodies grew and became more muscular all over, they sprouted more body hair as their skin became firmer, and mammaries receded in their chests, giving them flat pectorals.
Naturally, Ben got caught up in the former transformation, and he rapidly undressed to see himself change. What struck him as his body feminized was how this felt- irrepressible. Just as puberty turns someone from a child to an adult, this change turned people from one sex to the other, and it felt just as much a part of nature.
Also like puberty, the sensations came with a mix of confusion, shame and arousal. As his penis and testicles shrank away, he felt discomfort at his emasculation, but right after she felt intense pleasure as she embraced her budding femininity. To have her breasts grow gave her a deep-seated sense of satisfaction at the young woman she now was.
At last the change finished, and Ben- or Bernice, she figured- wondered how it had affected the world...