Yes it's sort of true - the concern here is about being susceptible to blackmail or coercion, which rules out pretty much anyone who has a personal secret which they feel ashamed about or would go to great lengths to keep secret. If you're gay and out, there's no security concern, but if you're in the closet, there is a chance someone could find out and blackmail you into revealing secret information.

This also applies to people with large amounts of debt - which IIRC is one of the most common reasons people are denied clearance. See here for more info (scroll to "Vulnerability to Coercion"): http://www.dhra.mil/perserec/adr/falsification/falsification...