@lovehydration
I want to point out that the "norm" is not something that is simply defined by mysterious "others", but you are right it of course depends on the boundaries of the system for which you want to find (not define) the mean/norm.
Now I do not assume, though I am no expert on this, that ployamorie/polygamie is not the majority case for relationships in japan.
There is of course an evolutionary reason that certain behaviors/social structures are adopted by a majority of the people, which DOES NOT automatically leads to deviations of the norm being detrimental in some way. I mentioned this in my previous post.
BUT it also doesn't mean that every deviation is automatically fine and dandy.
What the term healthy relationship exactly includes in the end I do not know. But from personal experience and observation I assume that these poly-whatever relationships are not something that would work for most people.
In the young and impressionable state the characters of this manga find themselves in, I even think people like that cause a lot of harm.
He should try to think about how the other person feels. Is she really ok with it or just playing along?
Now he himself may not have the maturity for this, but the sensei dude should have known better.
My actual point is, that I disagree with this notion of many "normals". The rhetoric used, in my eyes, tends to make the actual "norm" appear like the wrong choice. Like there is actually something wrong with you if you just want the "normal" (relationship, job ...) for yourself.
Don't get me wrong, I understand that in this story Meguro is young and she got screwed over so she has to get her bearing on how she actually should interact with people.
But I am still worried in what direction this will go.
I would definitely not agree if the behavior of her sensei is normalized through her in some way.