Not 15, not 13, not 12. In other words, by no stretch of any imagination were these two protagonists going get together any time soon. There’s no way the author was unaware of any possible “dangerous direction”— Mai was ten.
You really have no clue what you're talking about, do you?
This is so boring. Why do I waste my time explaining things to people who blather endlessly, as if they were hot experts, on some subject they obviously don't know half a shit about? In theory, a yuri forum should be more fun than this. I must be writing in the wrong thread.
There are scores of mangas where an underage girl who is an early bloomer and looks like a stunning sexbomb falls for an adult man and decides to seduce him. Not hentai mangas, shoujo mangas. Cute mangas for little girls who are expected to identify themselves with the ten-year-old heroine. And there's always a happy end, of course, otherwise the readers wouldn't be satisfied. The plots change a lot, but some key points are always there, like the heroine slowly wearing down the male lead's resistance with her sexiness, and the climactic moment where the male lead comes to the realization that he loves the heroine and age doesn't matter. In one story, the climax came in the middle of the wedding of the male lead with another woman. The heroine crashed the ceremony (The Graduate much?) and made him realize that she was the only one for him, so he dumped his bride and chose the ten-year-old.
If this sort of plot happens in shoujo manga, why couldn't it happen in a yuri manga for adults? And the answer is: of course it could! It just takes a mangaka who wants to make it happen. Sadly, it wasn't the case here: the mangaka was tired of this story and had started another series she wanted to focus on. So she just gave this manga a whatever ending that left two thirds of the characters hanging and didn't properly end anything. That's all there is to this.