At the exact moment December 31 2023 becomes January 1 2024, a portal opens to a parallel world where in the first decade of the 1800s, a big meteor exploded like a neutron bomb in the atmosphere.
But instead of neutron radiation, it released a type of radiation which normally doesn’t occur on Earth, that mutated humans worldwide. Roughly 8 percent of all cis people born since then have a syndrome that makes their sex and gender change in their mid 20s.
Once it starts, each time they fall asleep, they wake up having become a little less the sex they were born as and a little more the opposite sex. If for instance this person was born an attracted-to-females male, they end up an attracted-to-males female.