People aren't exactly forced to be promiscuous, they are raised to believe "everyone belongs to everyone else" so satisfying another's sexual needs is polite and expected. And a man and his mother leave the "reservation", a place where old cultures are left alone by the rest of the world. The man does fall in love with a woman from outiside the reservation but is later disgusted at the promiscuous behavior he wasn't raised in and so is disgusted by her.