It gets addressed. Hes male not allowed to use his male form, so he's stuck using the female form of a good friend. He changes it to be more masculine one to not be a clone of his dead friend he's borrowing the body form. And he can only change it so much.

So it's stuck in the middle. Basically a flat chested androgynous woman.

But because his new body is a genderless slime. Any human form he takes is basically just cosplay.

Rimaru is basically just doing magical crossdressing cosplay with the modified corpse of a very close friend.

The entire thing has very morbid subtext while also being really sweet.