Ok, I'll try to explain.
The same thing cannot be said with Tali. Her species aren’t monogendered; they do have males and females.
As for the asari, their species lacks biological sexes, so it's wrong to refer them as all-female.
They are monogendered -- but they are neither male or female. The reason we refer to them as females is because they are female-looking to us and it makes things easier for us.
The asari are an alien species, they are from an entirely different planet so their biology and society does not match the same rules as ours. Having a single gender existence means that gender as a whole has no meaning for them. To make it easier for humans and other two gender species to comprehend it is simplest to just call them all "female."
"Female" wouldn't mean anything to an asari. So "female" is just a label applied to the asari by us and other species that display sexual dimorphism.
The words "he" and "she" don’t match them, none of the things that are masculine and feminine match them. Even the word “it” wouldn’t match them.
To asari --> asari aren't female.
To asari --> asari are just asari.
So as someone already mentioned, what we are doing is called anthropomorphizng. And this isn’t a nice thing because if you follow Legion’s quote, which you can get during his loyalty mission, you will realize that this is racism. So whoever calls the asari female is being racist.
Sure, the codex does call them an all-female race but I take it as an error. The codex isn’t just wrong about Sovereign and the extinct rachni, it’s also horribly wrong when it comes to explaining about thermal clips.
Liara herself says that she isn’t a women or a female. She is an asari so why bother arguing against her. I take it she knows best.
And Casey Hudson said it himself. He is not totally wrong by calling the asari asexual because they do somewhat reproduce that way.
Or think about it this way -- imagine a monogendered alien species who are nothing like human women but instead they are very ugly abominations that can have sex with anything. Would you consider them female?
Asari are not female.
I hope this makes some sense.