There is, and should be, a critical difference between the civilian world and the military.
Individual vs. group/team.
Our American culture exalts, or at least used to in the important ways, the individual over the group. Our constitution limits the group and what it can do (though socialism has greatly eroded those protections) and spells out rights of the individual that the group cannot violate.
In most of civilian society, your individualism is not greatly compromised. We have worked hard at creating and encouraging group structures that foster equal opportunity, regardless of gender, race, sexual preference, etc. Most jobs can be done whether you are Black, White, Male, Female, Straight or Homosexual.
The military is a different story and must remain different if it is to do its best. The military can be likened to a team sport where the needs of the individual become secondary to the needs of the team. With one important difference. A team sport doesn't put its players in a position where they get killed on a regular basis.
The individual perspective on a job and a career looks at opportunity, advancement and eventually retirement. Are the opportunities for employment, promotion and the retirement benefits the same for all individuals? In most civilian occupations, those issues are not in conflict with performance and success of the corporation.
The military is, and must be different. And this is the problem. Society looks at military service as just another job. A career. And therefore, opportunities, promotions, and retirement benefits and goals are supposed to be equal for everyone. The rights and goals of the individual are elevated above those of the team. And that is the problem. The team goals and needs are distinctly different and if they don't trump those of the individual in necessary areas, the result is degradation of the military's capabilities. Degrade them a little and the mission suffers. It costs more in men and material to accomplish and in some cases, maybe it can't be done. Degrade them too much and then battles aren't lost, wars are lost. And maybe you end up being somebody's bitch!
In my not very humble opinion, what needs to be said, very strongly (and won't in this administration) is the purpose of the military. Our military's purpose is to defend the United States and its citizens, and impose our political will on others at the direction of our leaders. It is a career only for those who voluntarily subordinate their own individual aspirations to the needs of the group/team/military/state. There is opportunity, promotion and retirement available, but if the primary missions of the military are to be met, they can't be equally available to all. You can talk all you want about how being gay was no big deal in the Greek or Roman armies. Big deal. We aren't Greek or Roman. We are American. Greeks and Romans didn't have women in their military, nor did women have the societal rights of men. So why is it valid to pick out one aspect of their cultures and call them superior to ours and not the others?
Don't ask, don't tell, allows the maintenance of a certain illusion that promotes the group's goals. If a gay man wishes to serve his country and risk his life, maintaining a facade of being straight keeps the group cohesion. If a gay guy is admiring my junk or ass and neither I nor anyone else can see/notice it, how am I harmed? The answer is, I'm not. The falling tree didn't make any noise. On the other hand, for the same reason we separate men and women in the military, putting members of a group in close proximity to the group they find sexually attractive creates problems. Especially when the target group doesn't like it! And the fact is, there is no practical solution unless you throw out all restrictions and issues. More on that later.
If you put all straight men/women in a shower together, there is no friction. They don't want each other. Put in an openly lesbian or gay man and you have problems. Some straight individuals simply won't care. Their attitude is don't come on to me and we're fine. Another group is uncomfortable, but don't come on to me and we'll tolerate it. Continuing down the scale, others will be uncomfortable and don't consider it tolerable. And finally, you'll get a group who will see a come on when none was there and will be offended, maybe even violently.
So do you create a third accommodation? Put all the gays and lesbians together separate from the straights? I'm talking showers, sleeping and restroom facilities. This doesn't work either. Remember, males and females were separated in the first place because of the sexual tensions, desires, etc. Putting a bunch of gays together creates the situation you were trying to avoid in the first place.
If you continue this all logically, you come to three possible choices. 1) An all male, all straight military. This is the traditional, historical pattern. It has the advantage that it has been proven to work. 2) An all female, all straight military. It has never been tried, so you can't say it has never worked, but from a pure numbers point of view, we don't have the population base to get the required number of female volunteers, physically capable or not. 3) A mixed military. Male, female, openly straight or gay, with no accommodations for gender or sexual preference. Uni-sex everything. Showers, berthing, jobs, etc. This might work, but it is untested and unproven. It is popular with science fiction. And it goes against our societal norms, though that may be changing. At one time, coed dorms were a huge scandal. Now they are common. On the other hand, how many parents are going to be encouraging of their children joining a military where there is complete integration of all genders? How many women are going to want to join the military, knowing they'll be in barracks with men, sharing bathrooms, showers, etc?
I'm a historian. Which means you don't just study events, you study people. Machines behave logically (most of the time), people don't. Or more to the point, they generally behave like people in their society. If you don't understand that society and it's differences from you own, then you can't really understand why they did what they did, or what they will do in the future. It is pretty common in many parts of Europe today to have topless or nude beaches and kids are exposed to nudity at earlier and earlier ages. In that context, it is an easier transition to mixing facilities in the military and to a large extent, in many European countries, they have integrated to a much greater level in than in the US. And apparently successfully, though that apparent success may be just a lack of news over here. I say apparent, because they haven't really been tested militarily. No country in Europe has really fought a war since WWII and all the changes took place since then. We had some international cooperation in the Korean War and Australian help in Vietnam. The Brits have supplied the largest conventional components in Gulf War I and Iraq and Afghanistan. But they are the most like us so that isn't a test of a truly liberal military. Germany, France, etc., have fought small police actions in the former Yugoslavia and provided small contingents in Iraq, Afghanistan, etc., but they haven't fought a major war or had to commit major resources since they started becoming more liberal. So even for Europe, the more highly integrated military has never been tested.
Which brings me to my conclusion. The military is at it's best when it doesn't compromise it's team oriented needs in favor of the individual goals, i.e. women, gays, babies, long hair, dope smoking, etc. In so far as we have compromised to date, the military is no longer "best", but it is certainly better than "good enough". The integration of women in the military has created some problems and in some cases, a degradation in the ability to perform the mission, but we are still better than "good enough". We still win. If the goals of the individual ever become paramount to the degree that we are just "good enough", then we are at risk. Because it is a short step from "good enough" to "not good enough". And all it takes is one more step down the scale by us, or confrontation by a military that decided being best was better than being politically correct.