Its not widely known, but part of the review process for a security clearance is to see if a person has any history that could be used against them as leverage. Large debts, gambling habits, affairs, unsavory/embarrassing sexual habits, etc. These are all taken as signs that either the person has poor judgment or impulse control, or can be manipulated. It can mean being denied a clearance, or losing an existing clearance.

An example of why the government takes security clearances so seriously is Aldrich Ames. He was compromised by heavy debts and his affairs.