Protection?

The US military is reviewing its policy about banning openly gay soldiers. A study has found the risk "to overall military effectiveness is low" if the "Don't Ask, Don't Tell" law is repealed. It is surprising that the US goes off to fight for democracy in far off lands but has a problem with full disclosure. Why is it so? That "out" gay people would pose a security risk - or be too threatening to/for their compatriots? The reason for the original enactment of the "Don't Tell ..." law was not outlined in the report I read.