"So how is it that someone who received five deferments during the Vietnam War and thereby never served in the military can decide that any American willing to serve is unacceptable for military service?"
Because "that someone" is president of the United States, he has a phone and a Twitter account, and he'll throw GLBT people under the bus anytime it suits his mission of keeping religious conservatives happy.
"Where are the voices of military leaders (active duty, reserve and retired) speaking out against this ignorant, discriminatory, un-American action?"
They have spoken out. And the heads of the various military branches have said the status of transgender troops currently serving is unchanged until further notice and have said discrimination will not be tolerated. And Secretary of Defense Mattis has also put off any formal change for six months until a study is completed.
I think Trump's transgender action was repugnant, but he IS president. Elections have consequences.