The concern with gays in the military is the same with women in combat. And it's a legitimate one.
Two soldiers become lovers (be they a man and a woman or man and a man) and then the chain of discipline is broken. There is a risk that one will jeopardize his own comrades if the other is wounded/killed/in trouble.
You saw this in the Red Army during WWII. Men and women serve side-by-side, became lovers and then one killed his or her entire unit trying to save the other.
It's an intolerable condition. In the human species, emotion is stronger than reason.
Which, if Don't Ask, Don't Tell is rescinded would require gay men to disclose relationships--the Army would then have to break up the couple through transfers to other units. If the lovers are in different areas, they aren't likely to be able to do stupid things.
The same way husband/wife couples in the Army are never allowed to be in the same command structure at the same time.