And you're wrong about questionable topics being unpresidential. Many historical scholars have made note of Abraham Licoln's penchant for telling jokes, and some of them were quite bawdy for their time. Humor and the theater were Lincoln's main means of temporarily escaping from the horror he was living. That attempt to escape ultimately got him killed. In seeking humor, Lincoln was attempting to blow off steam. That's what the correspondent's dinner is about. All American's need to laugh on a regular basis, including the President. If what Laura Bush did was wrong, why was her husband, a good and moral man, busting a gut at her remarks?
Warren G. Harding once stated that it was a good thing he wasn't born a girl, "because I never could say no."
I agree with your premise that there is a line of dignity which an administration should never cross.
Laura Bush didn't cross it.
IMOHO