There is that, but as politikitty pointed out above ...You're already giving her access that walks right up to that line of keeping your countries secrets. Sleeping with her just brings up all sorts of questions that you'd rather be on a classified dossier between agencies rather than the front page of the New York Times.

As for infidelity, I suppose it depends on the infidelity.

Anyone who is up for, or getting thier clearance renewed gets investigated by the FBI, it's up to the principal investigator to make the "Vulnerability to blackmail" call. Hell I know that when I first got mine, an agent went around interviewing half my former co-workers, high-school aquaintances and even my ex-girlfriend. Whatever he got from them couldn't have been all rainbows and unicorns (I was a bit of a delinquent in my younger years) yet my clearance was eventually approved.