This was good... But I'm not sure why the mangaka bend over backwards to avoid Chiiko talking about why she did it...
Like, she felt sorry for him? And at that point she was in a good thing with Satoshi. I mean, if they were going through a phase, ok. But nothing... The event was something that basically determined the last arc of the manga and zip? Is it, "oh, she is broken and self destructive?" If so, she would do so many more things after breaking up with Satoshi. So the "why" is really important. And if you think about it, the only one that spells out "you sleep together" is Satoshi, nobody else does. What does that mean? Is it to allow readers to make up their minds? To leave space for 'they just made out and slept?"
I think it's a cop out in a otherwise good manga.