I'm having a weird and terrible feeling reading this, like the author is ready to bait us and hit us with Nagi actually being straight all along and just being gay because everyone assumed she is gay, like a reverse comphet
Has there ever been even one single yuri manga that did this without it literally being set in an isekai where everyone is gay lmfao. I can think of one manga where a straight girl gets sent to a gay isekai, but even then she bends like wet spaghetti in the end. And her not being into girls at the start was, y'know, from the start and not dropped on the audience as a twist.
The closest I can think is maybe that one "Happy End" manga or whatever it was called. It doesn't seem to be on the reader anymore, or if it is I can't find it. Where the whole story is basically the relationship between the two main girls and then in the final chapter one of them marries the others brother instead.