Originally posted by Sgt. Osiris:

Someone figured it out in another thread asking for the same thing. I never tried it and I don't know if it works for the current version of the game.

Originally posted by Higura Gainishigi:

So... is anyone going to actually answer OP's question or are we just gonna talk in circles?

Edit:
I did the research and figured out how to make it possible. Go into Shadows of Doubt > Shadows of Doubt_Data > Streaming Assets > Strings > English > Citizens. Open up the descriptors.csv and scroll down to non-binary and replace it with your preferred choice. Since outright removing it leaves a blank gender option. I assume you could replace the string with male / female but it might not identify them correctly. You could do what I did and change it to 'unknown'. Additionally you can go into Strings > English > Interface, and then select ui.interface.csv and scroll down to non-binary. Select the entire string and delete it. This removes the non-binary player gender selection entirely.
I'm sure removing they / them / their might also matter from citizen strings; but I can't be certain if its used when you have no idea if the killer is male or female. So I left it untouched in my game. Game works fine with the changes.

I hope that someone with some better understanding of the game assets can work out a better solution for citizens.

I guess I proved it is a game after all and isn't real life.