Well when he is 20 the sensei is 32 so no biggy
people have age gaps anyways
It IS a "biggy", right now he is a minor with a man who is 12 years older than him. Masahiro is still a teen!! They are in completely different states of their lives, Kousuke is the one who's helds all the power in their relationship and could (and does) easily take advantage of Masahiro. It doesn't matter if people have age gaps, those are between fully formed adults, not between a 15 year old and a 27 year old, that's simply (and legally) wrong.
Did you read Junjou romantica 3rd couple Miyagi x Shinobu miyagi is professor 35 years and shunonu is 17 (maybe) but i definitely remember there's 17 years gap between them so 12 years gap is not too much
It is too much still considering he is a minor, and no one said MiyagiXShinobu is an ok relationship, actually it is worse. Junjou Romantica it's bad, the only relationship that's somewhat acceptable is the 2nd one (and I don't remember much of them so I could be wrong), Misaki and Usagi have such a toxic relationship were is clearly stated how Usagi takes advantage of Misaki and literally r*pes him. All this just makes me sad, how come this are the mangas that turn into animes? Maybe you can blame JR as a product of it's time, but still, it should not be acceptable and simply ignored.

this is so good but i keep on thinking about how he’s 15 and he’s 27