This is, of course, only my opinion.
The Unites States Armed Forces exist for one purpose and one purpose only: To wage and win wars. Anything that runs counter to that objective lowers the effectiveness of the armed forces. Allowing gays in the military won't necessarily harm a war effort, but having them serve openly in a combat unit would be just as detrimental as having women serve in a combat unit. We don't allow that because mixing sexes provides an atmosphere of possible distractions that runs counter to the overall goal of creating unit cohesiveness.
The military should not be part of some kind of social experiment, though it appears that some folks think they can tinker with the system because "it's the right thing to do" in their spectrum of social righteousness. Their priorities run counter to the purpose of the armed forces.
Again, just my opinion.
Other veterans, what say you?