You're making this a gender war thing. It's not about who gets to dictate what to whom. It's about why something is dictated and who it's benefitting.
For genitalia, both genders are not allowed to show theirs. This is acceptable because it's fair.
For breasts, men can show theirs, yet women cannot.
For skirts: too short and a woman is labeled slutty. Why? Because people who sexualize them (often men) view them as sexy and wanting sex. And so these people then dictate that they should wear longer skirts so that they don't feel so guilty themselves. This ends up being one party dictating how the other should appear in order to accommodate their own needs.
Men can wear short or long. It's ok because generally women do not sexualize men in that way. But the point here is that the viewpoint of immodesty comes from the viewer, and women have no control in how they are viewed by others, therefore they should not have to adjust themselves to accommodate for something they have no control over.
> 1. Does a person have a right to show whatever part of their body that they want to? >> Sure.
No that is not my argument. I accept the laws that both genders cannot show their genitals or sexual organs. If we enacted a law that said all people have to hide their mouths I'd be in full support. We don't have such a law but we are close to it (face masks for covid). What I am against is unequal laws enacted by one group on another to accommodate the first or other group.
> 3. Should the rule be based on whether the viewer/displayer is female or male? >> Most definitely not.
This is not about male or female. This is about one group forcing their preference on another group to accommodate their own needs.