Given ambiguity in presentation exists as a consequence of biological diversity & medical intervention, observation isn't always accurate. Just ask the scores of natal women who have been harassed in public toilets for not being 'feminine' enough.
Outing violates fundamental human rights because it strips away personal autonomy, privacy, and safety. In communities with high levels of prejudice being outed can expose an individual to targeted violence, harassment, and hate crimes. Unwanted disclosure frequently leads to job termination, housing eviction, and complete social or familial ostracization. Forced disclosure can trigger severe trauma, anxiety, depression, and heightened risks of suicidality.
Deciding when, how, and to whom to disclose one’s identity is an essential exercise of human dignity and self-determination. Taking that choice away is a form of coercion and control.