The translator explained this choice in ch12's credits https://mangadex.org/chapter/60e13adb-407d-473d-b4de-f5746ad4007b/21 which basically comes from the 2 protagonist's perception of Niehara as the male teacher (the character the monster decided to mimic), this perception also works for Itou san to see Takahashi san as a girl and not a giant worm monster and by doing so, this becomes a yuri work... It is complicated.You really should switch to referring to the teacher with 'sensei' instead of "Mr".
The author made it explicit in previous chapters that the entity in question is genderless.
Constantly referring to it with masculine terms is both incorrect and messing with one of the themes of the work.
I understand this perspective, and I appreciate the constructive criticism, but I disagree. Itou and Takahashi use the same exact terms to refer to Niehara before and after learning they're a monster, so it's not like the work is explicitly drawing attention to such things. Furthermore, even if I had them use "sensei" instead I'd still have to decide on which pronouns to use for Niehara, any choice of which carries its own set of issues.You really should switch to referring to the teacher with 'sensei' instead of "Mr".
The author made it explicit in previous chapters that the entity in question is genderless.
Constantly referring to it with masculine terms is both incorrect and messing with one of the themes of the work.