They're mentally ill (as gender dysphoria is a mental illness as defined by the DSM IV, the last DSM that actually was worth anything, despite psychology being a meme), they're on hormones that make them unstable (estrogen for men is HUGELY disruptive and causes major depression, and the doses of testosterone that FTM transexuals are taking is enough to cause aggression, mood swings, etc.), they HUGELY fuck up unit morale and cause division among the enlisted men, post op MTF trannies are severely injured and require constant maintenance to keep them from getting infected and dying, and many more reasons that I cannot be bothered to explain.