This story is really ****ed up. The fiancee did make 0 effort to deal with his soon to be-wife, it seems that he's mentally disabled in some sort of way (maybe he lacks empathy completely?).
The general theme is the same we have in Goethes interpretation of the story of Faust.
We have a protagonist who wants to fulfill a certain desire through any means possible, even if it involves immorality.
The good thing is that we have a duality between "romantic feelings" and "lust", which is executed quite well compared to other manga which deal with the same theme but only explore one side of the medal.
In my opinion, it's not immoral to love someone younger (even if the difference is 10 or 20 or even 30 years) and even student & teacher can build a viable relationship, as long as both of them agree to not engage into anything before the student graduated.
It's even statistically speaking more realistic to find a potential spouse in-between your pupils, as they are way more in number and more diverse then your working colleagues.
You also know them a lot better than your colleagues.