yes it is a mental disorder
no it is a real birth defect
302.85 Gender Identity Disorder in Adolescents or Adults | The International Foundation for Gender Education302.85 Gender Identity Disorder in Adolescents or Adults
Gender Incongruence (in Adolescents or Adults) 
When it comes to matters of science, I don't care what the majority of people think. I care about what's actually true. Argumentum ad populum is a logical fallacy.
Freedom of speech is not freedom from criticism.