Flame Bearer

I could buy that but like you just said Byleth can't actually get romantically involved with any of them until the time skip when they're no longer students and in their 20's, and everyone shipping the characters knows this, so the student/teacher thing still doesn't make sense as a reason.

I don't see why so many people act abrasively about it. Even before we find out Byleth's real age we know he's about the same age as a lot of the students, and that fact is acknowledged by the characters pretty early on, and is one of the reasons why Byleth's position as a professor is seen as an oddity.

Honestly, most of this sounds pretty awful, some of it just flat out boarding on bad fanfic territory.

Is it really such a contested pairing? I ask since in general I have found it hard to find hetero Male Byleth pairings that don't include either Edelgard, Bernadetta, Lysithea, or to a somewhat lower extent from the other three Marianne. I might just be looking in the wrong places but I've found if I use Male Byleth without those characters I find myself out of luck.