There was an inner monologue at the end of chapter 74 by our protagonist, Kodama which he says that he would choose continuous joy over momentary pleasure. That statement, in essence defines the ending between him and Shizuku. It also didn't help the churning feeling in my stomach when Kodama and Shizuku started to use an abandoned house (THAT house from Ibitsu) for their meet-ups in the later chapters. They begin having full blown sex in chapter 129 and chapter 130 (final chapter) picks up from there with them going for several rounds until it's time for them to leave. Afterwards they say their goodbyes to go home, but as the Shizuku go to her apartment, she seems the shoes of a man already there. It seems he's been waiting on her; as she enters, they have an exchange of words before she gets in the shower and soon starts to break down. I don't know Japanese so I don't exactly know the context of what exactly is going on with her situation, but it's clear she hates whatever has to happen. I think the story may have even hinted at this situation because on the last page of chapter 90, Shizuku was masturbating while on the phone with Kodama but her ring (the ring she told Kodama several chapters earlier was a deterrent from marriage conversations) is placed on a desk in her room with a picture laid face-down. Some time later, Kodama comes to school only to find out Shizuku won't be returning. He end's up depressed at home with her pantyhose over his head to sooth his pain. His childhood friend, Akari comes over wearing pantyhose to try and cheer him up. It ends on a slightly comedic note with Kodama asking/proposing something to Akari in relation to the pantyhose and she reacts in an annoyed/disgusted way at it before the story closes out on their bantering. I think for the "momentary pleasure" might refer to Shizuku as she and Kodama can't see each other anymore. While the "continuous joy" is Kodama having Shizuku pantyhose to still reminisce about Shizuku.