@mommunism ignoring the grooming factor of having a relationship with your teacher the power dynamic an older sibling has over a younger can hardly be pushed aside as casually as your doing. Also equating family friends to a "blood relative" is reaching harder than Mister Fantastic from the Fantastic Four. Even if incest isnt criminalized its still heavily frowned up on so many social fronts, at least for a teacher-student dynamic (ignoring possible grooming) the two people are able to wait and till both parties are at consent-able age and their relationship will be more or less clean on all fronts. Also lets not forget this is talking about a teacher-student relationship which can also occur in higher education. Like in the case of this manga the dude came from 10 years in the future so he is not a kid in the strictest sense or what people refer to when they think of teacher-student relationship involving a minor and an adult. Its much closer to a college age kid having a relationship with a professor which still has moral quandaries written all over it but not as bas as incest.
On the incest side the relationship will NEVER be not looked down upon. Just look at the current president of France, he married his teacher who is the first lady and even if there relationship received flak it didnt deter him from being president but imagine if he married his sister lol. If you marry a blood sibling you will be a
social pariah if you celebrate it openly in most of the developed world. Hence why people who marry relatives(particularly in the West)are always given shit for it. Both are not healthy places to start a relationship but an incest one will leave you as a social outcast for life no matter the scenario, while in some circumstances a teacher-student relationship can be overlooked in certain circumstances.