Alex soon had an interesting idea- something that would massively change the world, but also keep it almost the same. He'd swap the sex of every person. Before having the device start the transformation, he made an alteration- all trans people's gender identities would remain the same, so their bodies would now fit their gender. He figured the change would make some people trans because they wanted their old bodies, so it would even itself out.
Wanting to see and experience the change for himself, Alex undressed and stood naked in front of his mirror while he set up the device. Once the change was set, the world started to transform. In perfect sync, 3.9 billion penises shrank back into their owner's bodies, and 3.9 billion vaginas sealed up. 3.9 billion people's testicles moved up inside them and started to reshape, while 3.9 billion people pushed what had been their ovaries into a new sac between their legs. 3.9 billion people's nipples enlarged and mammary glands formed in their chests, and 3.9 billion people's chests started to have the mammaries inside them wither away, leaving only pectoral muscles. 3.9 billion people's scrotums were sucked up flush between their legs and connected with a new tract inside them, and 3.9 billion people's clitorises lengthened and thickened into phalluses. 3.9 billion people who had had male bodies now had female ones, and 3.9 billion who had had female ones had male ones.
Unlike most people, Alex had not only gotten to experience it entirely aware of what was happening, but to get a kind of sexual gratification from it as it happened. As his cock receded to a tiny nub between his legs, he felt the nerve endings accumulate around the tip, and when his balls pulled up inside him, it surprised him how good it felt to be emasculated like this. Once his chest started to transform, the nipples swelling and the flesh under them rapidly feeling more sensitive, something inside her clicked. Like most people, he had considered his genitalia intrinsically connected to his gender, so- like about 98% of people to have transformed- now it was feminizing, her gender was changing too. She felt happy at seeing her large, soft tits, and an intense feeling was brewing inside her, the focal point of which was at her former scrotum. Staring at this part of herself in the mirror, she saw its centre under her clit slowly open, lips forming from the sac and moisture accumulating on them. Soon after she felt the feeling seem to reach her opening, and her vaginal tract connected her womb with her lips.
Like everyone else, and much to her disappointment, Alex came down from the high as soon as her female body finished forming. But unlike most people, she didn't go back to what she was doing, but continued looking at her new form. Naturally she remembered being a man, and so was surprised how weird it felt to be a woman now. Though she'd be lying if she said she didn't feel like she made a pretty hot woman...
She looked at the demographics to see they had shifted almost exactly- there were now slightly more women than men, rather than the other way round. She wondered for a moment whether to find out how the world had changed now everyone's gender was suppressed, or to try satisfying herself as a woman for the first time...