If you believe everthing the rabid anti-gay freepers post, I've got some real estate you might be interested in.
But the real costs are subtle and indirect. You will have a feminized culture in the military.
You do realize don't you that our men served in joint units with the British troops in the recent Iraqi war, don't you? You do know that the British military permits openly gay men to serve, don't you?
I can't justify abridging someone's rights based on speculation about what might or might not happen as a result of their excercising their liberty.