You not understanding the distinction between sex and gender is not my fault. To quote the American Medical Association's Journal of Ethics: "The terms sex and gender are not synonyms. Sex refers to the biological differences between males and females. Gender refers to the continuum of complex psychosocial self-perceptions, attitudes, and expectations people have about members of both sexes."
You can also refer to the statements I've cited from the American Psychological Association, the American Academy of Pediatrics, and Harvard University.