Someone once said: In Life, every person will either exercise discipline, OR experience regret. In this case, seems Kozue and Masato could not discipline themselves to keep their "horny" impulses in check until either a more appropriate time or a safer location. As a result, Kozue gets hooked on another guy's little friend, while Masato is to date blissfully unaware.
The vast majority of the time they involve blackmail schemes that a person above the IQ of 75 wouldn't fall for. If they were well constructed/thought out plans, theses stories would be much better IMHO.
Hmm...Netori?
Also, blackmail. Seems to always work, makes me wonder why they even agree in the first place. Although I suppose...The point of blackmail is that they generally can't refuse.
Well, though I suppose sexual compatibility is also an important part of a relationship. That, and also making sure noone is around when having sex at school.
That ending though. Well, really, she can make as many excuses up as she wants, but she had the chance to escape...Well, sort of? By denying the guy, and she didn't. So she betrayed him, and she can justify it all she wants but that fact wont change.