I've noticed a trend in the social justice left of treating someone's label or identity solely as an issue of self-expression rather than a way to describe or characterize people. Whether a label applies to a person is determined not on the label having any specific meaning and people an evaluation of whether that meaning applies to that person, but rather it's determined by whether that individual identifies with the label, and it's wrong to question their decision (it's often described as questioning their "experiences" or even invalidating their existence as a person!).

This happens most often for gender. In discussions on Reddit I've said that identifying as the other gender makes sense if you suffer from gender dysphoria (and a dissociation from your body/hormones/etc. that you try to fix), but not if you're just someone who doesn't fit into your gender role (because gender is not the same thing as gender roles—a tomboy or masculine woman is still a woman). I received some negative responses for this because it means that if a person identifies as the opposite gender specifically because of not fitting into gender roles then I disagree with them about their identity.

This basically defines man as "someone who identifies as a man", and woman as "someone who identifies as a woman", rather than "someone who meets [whatever descriptive criteria]".

This comes up most often for gender (because that's where questioning people's identities happens most at the moment, due to controversy over what gender means and the idea of people being transgender) but I've also seen it for other labels, especially related to sexual orientation (e.g. a man can be attracted to men and have sex with them but it doesn't mean he isn't straight, all that matters is whether he personally identifies as gay/straight/bisexual).

Does anyone have any insight onto this topic? Not about gender specifically, but rather the broader idea that you're not supposed to question or disagree with people's labels/identities. I'm interested in other examples, where this comes from, and whether you think it makes sense.

On one hand I can understand the idea that I can't dictate how you see yourself. But on the other hand, you can't dictate how I see you either. If you don't meet my definition for a label then I won't really see that label as applying to you, even if you use it on yourself. For example, if you identify as vegan as an act of self-expression of concern for animal well-being but you don't actually fit my definition of vegan of not eating meat or animal products (perhaps you eat meat once a week) then I won't really consider you a vegan.