It's kind of interesting that of all the things in this manga, what I'm reacting to the most is Shimada being called a boy. Compared to all the absurdities it really shouldn't be that big a deal. Expectations are powerful stuff, I guess?
Maybe Shimada is only the spirit and the body belongs to a woman he possessed. Or he can shapeshift at will like @Liquidxlax said.
I'm betting on the possibility that there's not any deep meaning behind it though. Hayashida is famous for having very abstract worldbuilding in her stories so a character like Shimada having boobs and being called a 'he' doesn't seem that farfetched.