Part of the process of getting a security clearance is proving you can't be blackmailed.

Lots of things you would think are disqualifying aren't, as long as you disclose them (being involved with BDSM is the most obvious answer I'm aware of).

The fact that they're LGBT isn't the issue with the repeal of don't ask don't tell. But if they're not out and they don't disclose then yes, this information is absolutely valuable.

Money stuff is another disqualifying event. If you're heavily in debt then you're vulnerable to being paid off in exchange for something against the interests of the United States.