I’m not convinced that they cannot change their preference, but I do believe that it is a disorder, a form of mental illness. The military should not have to accept the mentally ill. That’s not their job.
Anyway, I know for a fact that gay people have served honorably and with distinction in the service of our country, and I'm sure you do, too. More than a few of their names are carved on monuments throughout the country. But draftees were pre-screened, as were volunteers. Some managed to hide their preference. But if that soldier's C.O. ever suspected or found out, there used to be a vital understanding involved: shut up, keep it zipped and do your g**d*** job. Create any threat to this unit and you are gone (and "gone" could mean a stint in Leavenworth).
My fear is that now such necessary, life-saving discipline will be considered a "hate crime" in our brave new world.