It’s the first I’ve heard that to get a security clearance, you can’t be a dual national. So no employee with a clearance can be married to a non-citizen, or have been naturalized, even as a child?
Not exactly.
You are specifically asked if you have loyalties to another government/person/organization, and dual citizenship would indicate you do.
Access to classified information is limited, obviously, and even non-classified information or technology that is restricted due to ITAR is witheld from you, a dual national.
Being married to a foreign national is not an automatic disqualifier. It does make the adjudication process more complicated, as the threat to you is greatly enhanced: Say your wife has relatives in an unfriendly (or even “friendly”) country, and that country threatens the relatives unless YOU give up information. See? You may likely not divulge information, but granting someone a clearance with that possibility must be seriously examined. Being naturalized makes you an American citizen, you have taken the oath. Now, like before, you may be subject to blackmail/threats, so the adjudication process must be carefully done.
Hope that helps.