You realize it's called "appearance" for a reason, right? That means what you look like, or what other people perceive you as. Assuming or guessing that someone is what they look like is not stupid or a bad thing, it's called deduction, and everyone with a brain is able to do that. Even if you don't call the person a female, as soon as you see someone your brain makes a deduction to know more about the person. You might not address them as such because you don't want to be presumptuous, but if the person doesn't tell you otherwise, then your brain automatically deducts that they are what you perceive them as.
So if the person takes off their clothes and exposes genitals or body parts contrary to that deduction, you would be surprised.
It's like seeing an Asian woman with long, straight, black/brown hair, and assuming it's her's, then being surprised if she reveals it to be a wig. There's nothing wrong with it, it's only human.
Well, I've said my peace. If you still think it's stupid then I honestly wonder how you go about your life, but that's not my problem.
If 1 looks like 1, you'd think it's 1, you wouldn't just naturally assume that you're looking at the side profile of 4, and if your brain can't make that deduction then...well, you probably wouldn't understand even if I told you.
Dude... you're literally walking around with a bra on, the name Mana-chan and wearing makeup.
Obviously she'd think you were a girl