I mean, its weird as fuck.. But depending on where you live the character isnt a minor. Weird to see people get banned for saying they liked that part, when in Iceland you can be of legal age to date above 18 when you are 15. Now thats very young but in other european countries 16 or 17 is the actual norm.
But I guess this is mostly an American board stilp with American.mods upholding American values.
Not saying that dating a teacher isnt fucking weird, because it is. And if you have actual sex with her thats even weirder. But to ban ppl for saying they liked it, in a anime game focused on romancing various ppl.. well.. Dont look at any of the other japanese marker games like gal gun.
That's not it, it's not just the age difference that's a problem, although that certainly does not help at all.
It's the power dynamics that exist between teacher/student regardless. That like, age aside, between teacher and student, due to teacher being responsible for being in charge of a student's grades and stuff, there exists a huge power differential in teacher/student relationships and due to that even if a student nay want to say no to something or to the relationship in general, they night not feel comfortable doing so and hesitate or say nothing at all, in fear of potential repercussions like that.
Thus, regardless of age, it's in the teacher to say "no" in such situations to avoid them in the first place, and to do otherwise is extremely problematic and just let's them fester.
Then of course in top of that, there was Kamoshida's palace right before this in the game, which shows exactly how those kinda relationships can be abused, looking at what Kamoshida dud to Ann and Shiho in particular. Then there's how even in regards to just Joker, Kamoshida abuses both his position as a teacher/Joker's probationary status to try and get Joker expelled, which shows what a delicate position Joker in particular would be in such a teacher/student relationship (which is also something Kawakami herself is explicitly stated as knowing about in the lead up to Kamoshida's Palace, due to Kamoshida leaking the information around the entire school but also her just knowing in general, as is made clear when Joker first comes to school at the very start of the game and introduces himself to her/the principal and stuff).
Of course, on top if that, in the particulars of the Kawakami social link, you have to consider how it starts. With Joker, Ryuji, and Mishima going to some random apartment Ryuji happens to have access to in order to call some maid-fetish service... Only to be surprised when it's Kawakami if all people, their homeroom teacher who shows up.
At which point, Joker/Kawakami basically use this info to come to a mutual blackmail standstill.
That is, until Joker learns that Kawakami's only doing this whole sidejob thing in the first place because the parents of a former student of her's are blackmailing her for insane amounts if money and that's the only sidejob that's close to her school/home that also pays enough to meet their demands.
...At which point, Joker *shrugs* and continues blackmailing her into being his personal maid anyway, just ignoring that she's only doing that because she was being extorted in the first place.
And then as the social link continues, it's in no small part about what the role of a teacher should or shouldn't be, so when the time comes for the relationship option, it obviously references that and how that's super weird and the whole thing is in contradiction to the rest of the social link, but it just gives you the option anyway...
It's just bad on multiple levels.