Amazingly, those same groups do pretty well at controlling themselves now. Many units know that one or more of their personnel is gay, and yet, there really aren't that many cases where good order and discipline are interrupted now. And, repealing DADT doesn't really change anything down in the ranks. Most of the changes would actually be in paperwork and policy. There would be no longer a need to sign the page 13s about DADT and being gay. Investigations would no longer need to be conducted on if a person is gay. They may still come up on a gay person's behavior, and whether they are acting professional or not, but this investigation could be done on a straight person also. Repealing DADT would give the military members who want out before their time is up one less way to get a general discharge, could help the military on retention of priority jobs. It does not mean that a gay military person has to come out as gay to anyone in the military at all. And it doesn't mean that it will suddenly ignite some desperate urge for a flood of gays to try to get into the military. It does mean that the military personnel who are gay won't have to worry so much about the wrong person finding out and getting them put out.
And the whole argument that they should keep their personal lives personal is not always so easy in a military life. Very few, if any, other jobs do you live so close to coworkers, so very often many of your friends are from your job and not outside it. And generally, especially in military towns, you will see personnel from all parts of your chain of command in places where you frequently go to hang out off duty, including bars, restaurants, clubs, movies, mall, grocery store, or even the gas station. It's quite hard to hide the fact that you are a guy at a romantic movie with another guy on a date if someone from your chain of command happens to be seeing the same movie with his wife and sees you holding hands with the other guy. And its really hard to justify saying no PDA off base, out of uniform if heterosexuals are allowed to do it.