Somebody probably made similar arguments about pregnancy and other female health issues when women were allowed into active combat roles.
And specious arguments were made against gays and minorities before that.
As for numbers:
http://time.com/4874380/donald-trump-military-transgender-how-many/
What’s going to happen to them? Force them to give up their military careers? Honorable discharge? Do they keep their benefits? Trumpy opened one giant can of worms on this one.