Except it's not good regulation, it's actually detrimental to good order and discipline. If you'd seen half the scumbags that are allowed to stay in, but all the good people we've lost to DADT, you'd be appalled. I know many homosexual servicemembers, and they're among some of the most dedicated in our unit.
Plain and simple, it's the Pentagon and DoD not having any faith in its soldiers, sailors, airmen, and marines. They're so badly out of touch that they don't think we can put our thoughts on homosexuality, race, gender, ethnicity, and religion aside when there's lives on the line. That, or the vast majority of the DoD is highly homophobic.