Some of the fanwork...specially the comics and fanfictions. I'm not going to point out which ones but I see this a lot:

-Fionna likes Gumball
-Gumball rejects her (This usually is in a very out of character way they do it, making him come off as an ass usually or "leading her on" at first)
-Fionna is sad
-Marshall comforts her because he's always LUV'D HER.
-He admits feelings
-OUT OF NO WHERE FIONNA LIKES HIM AND FORGETS GUMBALL

Or replace all the above with Flame Prince (but they're actually together and he's being a jerk).

Or all the boys fighting over Fionna and Fionna is conflicted...

Or you know what? Best way to describe this is people writing as though this is some teenage romance or a shojo manga...

Why am I ranting? Because I never do and Fionna is probably one of the few female characters now days who is a STRONG female character and people just like to shit on that with typical romance bull. I'd like to believe Fionna is a bit more complicated than that.

Yeah ok I'm ranting over fictional character but whatever. It's late and I'm tired.