for me it was clear that in the end it was a monogamous relationship. The moment Takumi went back to their apartment and said "i quit", i believe he meant he quits girls. To begin with he was a playboy who just did it with any girls and got into trouble because of it but felt like it was normal to sleep with girls. Takahiko knew that which is why it didnt bother him knowing that Takumi slept around since it didnt have any meaning. The moment Takumi went back and hugged him, Takahiko hesitated a moment because Takumi was basically saying i just want you now... which means his waiting paid off and he finally got the one he wanted without coercing him into it.
Really like these one shots
I hated the third chapter. It's too sad and heart-breaking. Takahiko was obviously in love with Takumi since they met, and until the end, it was never clearly exposed that they got monogamously together, so he probably just kept suffering in silence.