If that's how you feel about it you are welcome to that, but from my perspective I think anyone should be allowed to lie about their gender, or rather than say lie, I don't think anyone has a right to demand to know your gender.
It's not a question of "rights." Mitsuki has obviously not committed a literal crime. What she's done is (a) deliberately misled another person (b) in a way she knew, or should've known, would cause them great pain if they found out (as we saw happen in this very chapter) (c) purely in order to preserve this pseudo-friendship pseudo-romance thing she had going on with her. What precise information Aya had a "right" to know about is irrelevant. Mitsuki hurt her. She did a mean thing. She should apologize! (And I'm sure she will.) And that's all I have to say about this.
If you take it upon yourself to make assumptions about me and cause yourself some emotional, mental, or other harm by doing so, it is not my obligation to apologize for your self-harm caused by your assumptions. It is not a "mean thing" done to others by not correcting their assumptions. Especially if you don't know if you'll be safe if you do so. The only person responsible for the thoughts and assumptions you come up with in your head is you. So the only person who needs to apologize for making assumptions is the one who made them, not the one they were made about.
This is ridiculous. Everybody makes assumptions constantly in everyday life and we require them to function. When I go to McDonald's, I assume the guy behind the register wearing a uniform is the cashier and is gonna take my order, nobody in the history of ever has asked that guy, "Hey sorry, are you the cashier?"
Sorry to say but 99% of the time assuming someone's gender works fine IRL and people do it constantly. Even if someone isn't gender conforming, they must be aware of this. Even the term non-binary in it implies the existence of the binary. Mitsuki is not wrong because she's not disclosing her gender, she's wrong because she knows that Oosawa thinks that she's a dude and is deliberately letting her keep thinking that she is one for her own benefit.
Phew. I thought this was closed as of the last few chapters, but since it's still ongoing, I need to say what's on my mind.
The issue here is not that Mitsuki has not disclosed her gender. It's that Mitsuki has not disclosed that the "two" people Aya knows are actually just one person. At-school Mitsuki is basically spying on Aya under a different identity. Whether she started doing it intentionally or whether it's because of Aya's assumptions is beside the point. She has concealed not her gender, but her identity so that she can interact with Aya as two different people. (whether she originally interacted with Aya at school or not is beside the point, she is listening in on conversations that aren't as vague as Aya thinks they are because Mitsuki has insider information.)