This is probably one of the military recruiting posters that gets the most giggles. It's from WWI, and it harkens back to the day when only men (and, in theory at least, only good, upstanding manly men who liked to sleep with lots of women, thank you very much) could join the military.
Of course we don't live in that world anymore, despite the efforts of some to push back against women in the military. We also live now in a world where people of all races can join the military (which took 100 years after the founding of the country). The military has the potential to be the great equalizer for all groups.
And this is as it should be. In a republic, military service is that great patriotic responsibility of all citizens. The ability to serve is one of the big markers of being a citizen and a participant in the state.
Traditionally, this has been the case with the US military in particular. Black servicemen helped to show the contradictions in both ante bellum Southern society and then the later Jim Crow South. (This is, of course, why black servicemen were a special target of both the KKK and lynch mobs trying to preserve the evil, idiotic systems.)
It has also helped to serve as the real melting pot and glue of our country in other ways. For many immigrant communities, it was fighting in the World Wars that helped accommodate other communities to their presence, as the sons of America died together. The same is true for smaller religious communities. The Army in particular has long held that all Americans, despite their religion, should be treated equally in their desire to serve, and even Satanists (the great boogeymen of the 80s to many) were acknowledged in Army chaplain manuals during Vietnam. It is also how many American traditions (such as turkey at Thanksgiving) have been transmitted.
All of this is prologue to my point: The ability to serve in the US military is more than just a symbol of citizenship. It is vital to the very nature of citizenship in a republic. This is why it is imperative for a nation like the US, which tries to maintain that citizenship is a widespread attribute of all born in this country (and a goal for any who try to move here) to not make stupid distinctions based on trivial matters.
There are certainly legitimate reasons to disqualify certain groups. A professional military does require certain skills and discipline, as well as people healthy enough for combat. It also requires people who are not likely to break discipline. But non-job related discrimination in the military of all places is a wound in the heart of American citizenship.
This is why both DADT and the ban on women in combat roles bother me so greatly. It denies one of the central parts of citizenship on people for objectively discriminatory reasons. It serves as nothing more than a denial of full citizenship to women and gays. And it makes me very happy that Mullen, Gates, etc. are working on ending this. I hope it is accomplished within the year.