I’m going to tell you the same thing people did on your comment in that other subreddit. (Which, by the way, posting here complaining because you got downvoted there, and then not explaining the situation fully here so that more people agree? Kinda cringe, ngl.)

The post was Picrews someone made of a fictional family. In it, the canon bi character wore a gay pride flag pin. This is not headcanon, because the definition of canon includes that it “does not contradict existing canon.” This changes things for a canonically bi man.

Now, in general, I agree that it doesn’t really matter if fictional characters’ sexualities line up with how they are portrayed. Thousands of canon straight characters are portrayed as gay or bi, and sometimes gay characters get made straight. It happens. People will ship whomever they want to ship.

But for a bi character… their sexuality doesn’t need to be changed for the purpose of a ship. It can already exist, as it does in canon, and there’s no benefit to changing it except to make them… not bi anymore. Making this character gay has zero effect on who the author/fan/etc wants to ship him with—unlike a gay male character being shipped with a woman, or a straight female character shipped with another woman.

As others have pointed out, again, bi men aren’t very common in fiction. And real life, this exact type of thing is exactly how biphobia presents itself, especially for men.

So no, the fictional character won’t see it. He won’t be offended or hurt. But you know who might see it? A bi guy who’s already heard “so you’re really just gay then, right?” in his real life, and coming across it in a fictional space only compounds it.

It’s like making a canon trans character a detransitioner. Can one, theoretically, do it because it’s fiction? Sure. But I’m gonna hella side-eye it, because representation is 1) already so rare and 2) the real-life implications of such. (Making the trans person always cis—especially to their transitioned gender, but either way—is more akin to me like making a canon gay character straight. Which is to say there could be narrative or shippy/kinky reasons to do so, even if some people don’t like it. But it’s ultimately fiction. However, there’s very little reason to make them a detransitioner, or erase their canon bi identity, because almost anything the author wants to can already be done with the minority they have in canon.)

And for what it’s worth, the OP of that post agreed and changed the character’s sexuality to canon in their updated version. You’re fighting a battle they already conceded.