They're mentally ill and the mentally ill don't belong in the military. Enough said in the matter!!!
Credible source:
Title: Johns Hopkins Psychiatrist: Transgender is ‘Mental Disorder;' Sex Change ‘Biologically Impossible’
Snippet from the article...
"Dr. Paul R. McHugh, the former psychiatrist-in-chief for Johns Hopkins Hospital and its current Distinguished Service Professor of Psychiatry, said that transgenderism is a “mental disorder” that merits treatment, that sex change is “biologically impossible,” and that people who promote sexual reassignment surgery are collaborating with and promoting a mental disorder."
http://www.cnsnews.com/news/article/michael-w-chapman/johns-hopkins-psychiatrist-transgender-mental-disorder-sex-change