How common are Bisexual heroes in Fimfic stories?

Depends on what you mean by bisexual, for example:

1. Does the main character acknowledge that certain people of all genders are attractive?
2. Do they act in any kind of "flirtatious" manor (I mean that in a very loose way, like they wouldn't outright reject someone's advances, but not necessarily make any themselves) towards all genders?
3. Do they actively seek out/flirt with all genders?
4. Do they date/have dated all genders? (Regardless if they initialized them)
5. Do they actively engage in mature acts with/have inclinations to do it with all genders?

Because it could be that all characters are in some definitions bisexual, just have only openly engaged in relations with one being/gender, so there's no telling how open they are to other possibilities...

But as for any character that has canonically engaged in relationships with more than one gender, romantic or sexual, I can't honestly think of any off the top of my head, seeing that most places frown upon homosexual relationships.

I can only think of one or two and one of em was an Anon story, so i'd say uncommon