I am a massive Flayn fan and I love seeing romantic art or fanfics of her and Byleth together, but this is sadly a pairing that either goes ignored or is openly vilified by much of the FE fanbase*. Mind you, this same fanbase has no issues with romantic Flayn pairings in general (Flayn x Dimitri seems to be wildly popular for some reason) but only is it when Byleth is involved that people seem to get up in arms. Yes, there are people who hate characters like Edelgard and Rhea, but nonetheless there are tons of art and stories of Byleth with them. Flayn by herself doesn't attract nearly as much vitriol (that I'm aware of), yet she's one of the characters least represented in Byleth ships (and yes, I know there are others, no need to bring it up). Flayn fans are also routinely mocked and insulted, and whenever Flayn x Byleth content IS created, it's always met with the same derision and backlash, often couched as "memeing". This is grossly unfair to Flayn and her fans, and both deserve better treatment.
I know what many peoples' first objection is going to be to these points: "Flayn is a young child! Loving her or wanting her to be with Byleth is wrong and you are sick for it!" While this amounts to nothing but an empty and unwarranted personal attack, there are three reasons why it's completely invalid:
Flayn is not a child or "loli" as people often label her. She has a teenage physique and is literally coded as being of 17 years of age (I'm not going to bother going deeper on this point because detractors will accuse me of "fixating on her body")
Even disregarding the first point, no one is ever given hate for pairing Flayn with other characters as I've mentioned multiple times now. If Flayn is "too young" to be romantically involved at all, why is it still acceptable to pair her with Dimitri, Ferdinand, Linhardt, etc? Why is it ONLY with Byleth that it's a problem?
People say they don't like pairing her because she "looks too young", but that doesn't stop the plethora of Bernadetta or Lysithea x Byleth content being produced, as well as the majority positive reception it gets in contrast (and yes, I do mean pre-timeskip).
In summation, I am one among many passionate Flayn fans and we will continue to be her fans no matter how many insults and memes people bombard us with. Flayn is a kind, sweet and wonderful girl, and she deserves way more love and respect than this fanbase gives her, and so do we.
*To be clear, I'm not referring to NSFW content of any kind, so don't even try to throw that argument out. If you have to make this about sex, that's on you.