In a dystopian society, at 18 you get a slip with the name of your soulmate determined by some council. No one questions it and everyone pretends like the system is good, ignoring obvious red flags and various little inconsistencies. Maybe some people get divorced, but that’s okay, it only means they married the wrong John or Jane - stuff like that. If you dare question it you’re a pariah, of course.
The horror can come in many forms. One example could be suddenly being given a name that means you’re gay like in the story, but this time make it an actual horror scenario where the person is gaslit into believing they’re gay instead of this “the council knew he was gay all along” bullshit, but reddit wouldn’t like a story like that. Of course the protagonist had to have actually been gay in denial all along.