Hope you find it!

What I said in my Tumblr post was just what anyone can find with a bit of research, the plain description of what it was, not my opinion. I tried to reach for the closest case I could think of the scenario you proposed, but it was an scenario I never claimed so I don't really understand

Argue, that Class-S never existed? Or that the outcome was frowned upon by the protagonists? I never claimed they were happy, but the dynamics existed for a purpose and that is something I did not make up myself

And as for your other post, yes, all of this is just stained on the dynamics on Class-S, Maria-sama ga Miteru is well ahead of it, that is why some dynamics change, but the general dynamic of "this relationship happens in this school" is also obvious.
I do not really get what you are trying to

Sei is literally studying in the same campus as her little sister and the girl she see is the one is not her little sister. Yoko only sees Sachiko when her grandmother dies. How are they maintaining a relationship? I really do not see it.
Sei does not stop being a lesbian, you are absolutely right

A more recent example is the dynamic in Maria-sama ga Miteru: the romantic relationships between the female students are only allowed when they are students, they don't even consider maintaining the relationship with their "sister" once they graduate, like it was a rule.
Good question, really!

in love with (Rei) (trying not to spoil it for you in case you're not familiar with), after the tears, everything becomes straight for everyone, someone actually marries a man on the screen and Nanako says she is happy and hoping to find a man for herself soon. The general tone changes drastically.

Although the men they will marry do not tend to appear in the stories since that happens when the fantasy of youth ends, the first example that comes to mind is "Oniisama e..." (Dear Brother) by Riyoko Ikeda. After it has been made impossible for the protagonist, Nanako, to be with the girl she was