Bitter Regret

Yeah. Maybe there's something about how that's viewed in Japan that makes it worse, but it sure doesn't seem like something that could actually be used as blackmail material - especially when it was her fiancé. Maybe if she'd been caught trying to kiss someone other than her fiancé, or if she were actually engaged to a student instead of a teacher and keeping that hidden, or if it were sex instead of kissing, as you suggested, then it could work, but as it's shown here, it makes no sense.

Of course, no matter what she was caught doing, it was just whatever ploy the mangaka came up with to get things going and isn't terribly relevant to the rest of the story, but it should have been pretty easy for the mangaka to come up with much more plausible blackmail material.

Pretty sure seiji is a teacher.

Eh, knowing the hentai formula, the fiance will see it the first time, gets hard and jacks of to the pics/vid, realize she's beautiful when she's getting the satisfying dicking he can never give compared to the chad, and becomes the voyeur cuck as someone else impregnates his fiance/wife for life.

That's what I did days ago but I decided to read it thru now. Should have followed my first inclination.