Thanks Sam!!
The supremes have ruled (I can't find the case; sorry for not giving a citation) that clothing must cover certain parts of the body. Namely, the anal cleft and pubic area. Women must cover the airola and nipples of their breasts (unless a women is in New York City; a judge ruled that under the equal protection clause, women don't need to cover their breasts if men do not have to).
Police are not supposed to be judges of pop-fashion. Police are to ensure that a person's private parts are covered, AND THAT IS ALL!
Therefore, wearing tighty-whiteys, boxers, or panties is legal.
Sam's actions are bigger than a TSA tussle. This is constitutional stuff--more than the illegal search and seizure stuff (4A). This is also a 1A issue.
markm