Okay, I personally am serving for the U.S. Army. So here's my standpoint:

I have nothing against transgender people, although being a transgender person in the military (in any branch), can cause some issues.

1.) Urinalysis drug testing is often done once a month. When drug tested, you have an observer of the same sex watch you give your sample. Now, knowing you can "identify" as a female but still not have gone through the actual surgical procedure, means you can be a male, with a penis, identifying as a female, being observed by a female.

2.) Showering in the field: When deployed or in a field training exercise, you will shower with that of the same sex. Scenario: Male identifying as a female, but still hasn't undergone surgical procedures (still has a penis), would be showering with females. Keep in mind, a 3rd shower group for transgenders is called segregation.

3.) PT Tests, or physical fitness tests: These have to be done at least once every 6 months, (at least for the Army). For those that don't know, these tests are scored differently by sex. Females have lower score requirements than males. What if a male identified as a female solely to score a 300 (Max score)? Also: females are exempt from PT tests if pregnant, but what if a transgender female, now a male, got pregnant?

4.) Deployment: similar to issue 3, females are undeployable if pregnant, but what if a transgender female, now a male, got pregnant?

5.) AR 670-1 (appearance regulation article). This is the article that says males have to shave, have tapered hair cuts, tattoo regulation, etc. For example: Imagine a transgender male, now a female, growing facial hair. Who would tell her to shave her face?

6.) Lodging is separate for males and females when in training. See issue 2.