In 2011, the repeal of "Don't Ask, Don't Tell" allowed gays, lesbians and bisexuals in the military to serve openly. But the ban on service by transgender people continues, because it is based on military medical regulations put in place before the American Psychiatric Association declared, in 2013, that being transgender is not in itself a mental disorder.

Last July, President Obama signed an executive order "prohibiting federal contractors from discriminating on the basis of sexual orientation or gender identity." But that does not apply to the military.

"It was a Command Sergeant Major in Afghanistan who pulled me to his office and he said, 'I need to know exactly what you are,'" recalled Landon Wilson.

Landon Wilson LANDON WILSON

What Wilson was, was transgender. In his case, born biologically a female but living as a male. His commanding officer uncovered his secret at the end of 2013 while reviewing then 23-year-old Wilson for a promotion. He was forced to leave the military with an honorable discharge. He says he was worried about his buddies and the mission.

"My main concern was who was gonna take my spot?" Wilson said. "When you're in a place like that, you can't really afford to lose anybody."

Wilson had enlisted as a female then decided to transition. That meant beginning to live and express himself as the gender he identified with. He took male hormones and by the time he was deployed to a new unit with military intelligence in Afghanistan, he looked and sounded masculine and was placed in a male barracks.

"It was the best experience of my entire military career," said Wilson. "It was probably the only time that I knew 100 percent that I could focus on my job without worrying about my gender coming into play."

Mara Keisling, who transitioned 15 years ago, is the executive director of the National Center for Transgender Equality. She says a transgender person is somebody who knows early on, often from their first conscious moment, that their given gender isn't right.

Mara Keisling, the executive director of the National Center for Transgender Equality CBS News

"Transgender people are people whose gender identity, that is their internal sense of their gender just doesn't line up with what the doctor told our parents when we were born a boy or a girl," said Keisling.

From the age of 3 or 4, Wilson felt his gender was male.

"I remember announcing proudly to my mom that this whole girl thing just wasn't cut out for me," said Wilson. "And I recognized then that the reaction I got wasn't probably the best."

Wilson says the overall message he got was that "this is how things are and this is how they're going to be." A 2011 survey by the National Center for Transgender Equality found 41 percent of transgender people had attempted suicide. The decision to transition can bring a sense of peace.

"There's a relief in saying, 'you know what? This is who I am,'" said Keisling. "There's a relief in saying, 'this is what i'm going to do.'"

Transitioning may or may not involve hormones or surgery. Wilson says, in his view, the procedure is irrelevant because it doesn't change who you are as a person.

"How we see ourselves and how we present ourselves to the world is much more important than the underlying layers of what's under or clothes or what could be under our clothes," said Wilson. "Gender is completely independent of your sex."

It is difficult for people to understand that a person's biological sex can be different than his gender. Ignorance about that has led to discrimination for transgender people in all walks of life, not just the military. As for Wilson, he says that if he could re-enlist, he would do it in a heartbeat.