Sheep is right in one respect, gender is the state of having either XX or XY chromosomes. However one's perception of their gender IDENTITY is a totally different matter. He probably thinks whoever thinks they're female if they're male and vice versa is mental. In which case, he is incorrect. Although I wouldn't blame him for it, in a black and white view of the world, it would look that way.
I support my claim with a study from the American Psychiatric Association, which includes this portion of text.
"DSM-5 aims to avoid stigma and ensure clinical care for individuals who see and feel themselves to be a different gender than their assigned gender. It replaces the diagnostic name “gender identity disorder” with “gender dysphoria,” as well as makes other important clarifications in the criteria. It is important to note that gender nonconformity is not in itself a mental disorder."
Source Link:http://www.dsm5.org/documents/gender%20dysphoria%20fact%20sheet.pdf