No one knows why it happened, or how it happened. But one day, at the same time all over the world, every single human changed sex.
People sometimes talk about events where 'everyone knows where they were when it happened', but this was very literal. People all simultaneously paused what they were doing, recognizing that something felt very weird. Even people who were asleep were woken up by the sensation. They knew they weren't ill, but their bodies felt odd. Some people even undressed due to the skin irritation of what had started to happen to them- those people got a front row seat to the changes.
And in perfect sync, half the world started to feel or see their bodies metamorphose in one way, and half saw them transform the other. The half born with masculine bodies developed soft curvaceous figures, formed soft breasts on their chests and had their penises shrunken away to be replaced with vaginal tracts and wombs. The half born with feminine bodies developed firm figures with more muscle, their breasts shrank away to toned pecs, their pussy lips sealed up and testicles moved inside them, and their clitorises swelled and thickened dramatically into phalluses.
Sexualities and genders were transformed by this too. Anyone who had been straight before was attracted to the opposite gender to the one they had become, anyone who had been gay before was attracted to the same gender as the one they had become, people who'd been trans before were now the gender they'd always wanted to be, and a few people were now dysphoric about their new gender, though most adopted to it. (Bi, ace etc. people were unchanged.)
So, who will this story focus on? You choose... (A/N you're more than welcome to describe their perspective of the change if you like!)