Why is the army so anti-gay?

What is so threatening about gays in the military? Is it that the guys won't bond as well as a fighting unit if there might be a gay in the bunch? Being in California near San Francisco - I've worked with gays for years professionally. If you don't bother them they don't bother you. Never been grabbed or propositioned once! And some of these guys are some smart. So why at a time when recruiting is so difficult would the army see the need to exclude people from its ranks?