First, the rational people don't realise anyone cares about sex. After some public incidents which embarrass all the involved today-people, they figure out that something strange is going on, though they have no idea what and don't understand embarrassment. Next, they are too distracted by burning curiosity to have any sex, and ask incessant questions about the customs regarding sex. Then they decide to try out the customs.
Nirit asks his friend Keatac to marry him. They have a ceremony with some authentic lines from romance movies. They treat it like acting and try to stay in character, but no one understands their character's motivations very well. The part about the audience sitting and watching silently is especially odd: it seems boring and pointless. They decide not to bother with that part, and the most members of the audience spend the wedding in conversations with their neighbors. The happy couple get a hotel room and lock the door to keep all their other friends out, then dutifully wait for nightfall. At sunset, they finally have sex. They gather they are supposed to stay with the same person for a long time, and be exclusive, so they try to.
Previously, Nirit had frequent sex with his friends Keatac and Syl. Nirit fends off questions from a slightly annoyed Syl. Syl asks why Nirit chose Keatac, and Nirit tries to act in character by saying Keatac has pretty eyes. Syl watches some episodes of Friends, then lifts weights for twenty minutes and propositions Nirit again, saying they should have an affair now. Nirit doesn't understand much, but he's convinced that's against the customs no matter what Friends depicted, so he finds some Christian websites to back him up. Syl then starts asking about the purpose of the customs, and Nirit replies that he isn't sure but wants to try them. Syl gets bored with the argument but is quickly distracted by a new hobby: paintball.
Eventually, Nirit misses sex with Syl and researches what he must do to have it. He discovers breakups, and (mimicking a TV character) tells Keatac that it's over. Keatac asks for some time to look up the proper reply. The next day, Keatac says he's ready, then shouts that he hates Nirit and exits the room by stomping his feet loudly and slamming the door. Nirit claps happily; he hadn't thought to try to damage a door for no reason, but recognises good acting when he sees it.
This begins an epidemic of breakups. None of the people broken up with mind. During the entire experiment, no one lies, and they only cheat out of ignorance or to try it out. There are few breakups by phone, despite the convenience, because they try the monogamy custom seriously and realise that it is dishonorable to breakup without meeting the person face to face.
Before long, people make a habit of breaking up immediately after sex, so if they run into someone else and want to have sex, they won't have to find their last partner to breakup. Getting together is just a matter of saying they want to, and unlimited breakups with the same person are allowed, so they find this makes sense. It takes Nirit a while to figure out how monogamy actually manages to exclude any sex: at first he thinks that surely the people of today, who've had this system for millennia, must have already figured out to breakup after every sexual encounter. When Nirit tries to explain the insight to a person of today, the person comments sarcastically that college kids already figured that out. Nirit doesn't understand sarcasm, but isn't surprised that young people are ahead of the game.