Simply put: Open military service today... full equal rights including same-sex marriage tomorrow.
I consider myself a lefty/progressive populist in good standing who is both anti-War and anti-American Imperialism, somebody who is 90% in agreement with Chalmers Johnson and the American Empire Project.
Over the years some of my fellow travelers (lol) have objected to the left taking up the cause of the repeal of don't ask don't tell, viewing this more as a pro-militaristic "equal wrongs" then "equal rights." A good example of this was expressed in a diary yesterday.
However, regardless of one’s views on the military-industrial complex, fortress America, and our imperialist role with bases in hundreds of countries all over the world, the fact is (whether it should be the case or not; alas) that historically open and acknowledged military service is the route by which oppressed minorities eventually gain full and equal legal, civil and social rights in America.
The fact is that was important at the time for abolition and (albeit briefly) reconstruction that African-Americans served in the Union Army during the Civil War.
It has been an important path to respect, lessening of social discrimination and equality for every immigrant group throughout American history, from Germans and Irish during the Revolution and War of 1812; to Jews, Italians and other southern and eastern Europeans in World War I and II; and again for African-Americans and the civil rights movement coming out of World War II and Korea.
Military service today... full equal rights including same-sex marriage tomorrow.
That is the real reason why it has been fought for, for so long (regardless of one's views about the military and American militarism and the military-industrial complex per se)...
...and by the way, this is the real reason why they are against it.