My person current [and likely very silly] theories about the actual nature of the show:
1) Somehow, someway, Alice (MC's girlfriend of several years who broke up to him 2 years before things started) has orchestrated the whole thing. Why? Who knows. But between how much information was given about her backstory, how MC obviously still cares for her, and the fact that nothing was said/done about her character to make the player be put off by her (she didn't cheat, etc), I feel she will likely come back into the story at some point. Her being behind this, somehow, is one such way.
2) 2) Evelyn is the one who set it up/the one who knows something. She's rich, lonely, and wants a real relationship. She's also highly sexual and encourages you to be with the girls. Poly relationships could be a kink of hers, or even if she is as straight as she says, she might like seeing men with other women (provided it isn't cheating on her). Lastly, as she has some concerns that she is only wanted for money, she may feel that offering to share a man with other girls (that she approved of when casting for the "show") that she was offering something else to appeal to someone.
This would mean she cast MC as well, having probably done some research to make sure he was a decent guy, had the looks she wanted, was wealthy enough to not be interested in her for her own wealth, etc.)
She could arguably have the resources to set the whole thing up, as well as the surveys the girls had apparently taken in the past. Maybe Katherine is a friend of hers (or she knows through connections that her spotlight is falling, and she's lonely, and simply decided to involve her as well to see if she could spread some happiness.
Hell, if she is that wealthy, that crazy house could even be hers, or one of hers. As for the other guy? Having him on adds to credence that it is a reality show, and with him being an oddball (though harmless) it takes attention away from her when people think some things are odd.
3) (weirdest and least likely, though some things point to it) The house is a survival shelter of sorts. Maybe it's an academic experiment (could these people survive together if they were forced to, without jealousy pushing them apart?) or perhaps something more sinister (Aliens/nukes/zombie apocalypse are known to be imminent, and a group of survivors was chosen to repopulate). I feel this one is WAY WAY WAY too out there, unmatching the game's tone, and the least likely. But it also could explain the clandestine nature of how the show was set up, why they were each chosen, why the underground is so huge and so well stocked, and why Katherine, in truth or dare, will assure you (should you pick the option to ask about it) that you are all, indeed, safe in the house.
