It's a fandom wide problem. You'll see it in any fandom.
The issue isn't whether people prefer that in real life, it's the way it's written in fandom because it relies on really negative stereotypes and sometimes edges into fetishizing. Assuming someone bottoms or tops because of their emotional state or physical appearance is problematic. Changing someone's characterization to make them better fit the stereotypical idea of a top or bottom is also problematic.
Re: Can fic writers specify who tops or bottoms?
It's a fandom wide problem. You'll see it in any fandom.
The issue isn't whether people prefer that in real life, it's the way it's written in fandom because it relies on really negative stereotypes and sometimes edges into fetishizing. Assuming someone bottoms or tops because of their emotional state or physical appearance is problematic. Changing someone's characterization to make them better fit the stereotypical idea of a top or bottom is also problematic.