Don't Ask Don't Tell has been declared unconstitutional. Can we have a civilized discussion about it?

In a New York Times article about the ruling, reporters quoted the judge as saying that the government had failed to show how banning gays in the military was “reasonably necessary” to protect “a substantial government interest.” The judge also wrote that the policy violated 1st and 5th Amendment rights of gay service members.

Are gays in the military a threat? If so, how? If not, why not allow them to serve openly?