Dale Carpenter explains it:
Whether it takes the form of saying that some recruits will refuse to enlist, or that some personnel will not re-enlist, or that some will get heebee jeebees in the showers, or that some will not obey orders from gay commanders or share tasks and objectives with their gay comrades, it comes to the same thing. Gays are to be excluded, not because of their own merits, but because we fear that some people around them might not be able to handle the truth. It is not a judgment about gays at all, but about heterosexuals.
This dreary picture of heterosexuals is falsifiable. The available evidence, including ample experience in our own and other militaries, strongly suggests it is built on myths about heterosexual frailty and irrationality.