It's entirely possible that Hanabi and Mugi only fell in love with their respective partner because they were the first ones to give them that affection. But it becomes a question if what they felt was ever love in the first place when they can so readily find someone else as a replacement, sleeping with anyone who shows them even the slightest affection. Love is a tricky thing, after all, and teenagers are at their most confused stage in life. They haven't the slightest idea of what they want. They act nonsensically at times, and can change their attitude on a whim, giving up on their love at the end of one episode and then cheerfully encouraging each other to confess at the beginning of the next. Most of Kuzu no Honkai's cast is comprised of hypocrites who get upset with their partner(s) for acts they have been committing themselves. Love is blind, as the cheesy proverb goes, but in this case they must also be blind in the literal sense. The real victims are Norumi (the male teacher) and Noriko for being deceived by such deplorable people. And so I cannot bring myself to care when it tries to play Hanabi as the victim, the tragic heroine.
They deserve everything that happens to them. It becomes impossible to feel any empathy for them and the grief they are constantly plunging themselves into. But it escalates, and they, and just about everyone else surrounding them, wants more, more, until nothing remains in their hollow lives but sex and sleaze.
KUZU NO HONKAI SANAE SERIES
If that is where the series stood, all may have been fine. And so the cycle, their tedious lives continue. But they still can't give up on the one they love. They can't love each other because they love someone else. Hanabi and Mugi are lonely because they cannot be united with their loved ones, and so they seek to find some solace in each other. There is seldom a character within Kuzu no Honkai who behaves sensibly or is capable of thinking with any other organ except their genitalia. If you found anime such as Aku no Hana and School Days difficult to watch, you might have yourself something of a challenge here, too. It is misery for misery's sake, existing almost exclusively to make you feel like crap. Kuzu no Honkai doesn't have that message. There exist a wealth of fiction that, while depressing, and perhaps never even enjoyable, are still valuable for the message they are trying to make.
It isn't necessary for a story to make you feel happy. The characters are relentlessly trampled upon, until, indeed, as the title might suggest, they are reduced to human trash. It introduces itself as a melancholic tale of unrequited love, and quickly transforms into something far more ugly. There aren't many anime as uncomfortable as Kuzu no Honkai.