An eternal question

There's a late 1975 M*A*S*H episode with a scene in the mess tent, when Hot Lips and Frank come from the chow line to sit at a table where Hawkeye, B.J., and Potter are already sitting. This was the first year B.J. and Potter were on. As the two majors walk up B.J. gets to his feet, and Hot Lips notes unpleasantly that at least one of the three male officers are mannerly enough to rise when a woman approaches. B.J. rebuts it was just that his short were riding up. I've always wondered about this one.

Remember that B.J. was still the new kid on the block - he'd arrived a week before Potter did (that is, both out-text, in the episode before Potter did, and in-text, per a line of dialog of B.J.'s in The Bus) but Potter was a longtime veteran of assimilating into new army units. That may mean that B.J. still had civilian, civilized reflexes enough to rise for a woman as was still done socially in the 50s. And it may mean that, having it drawn attention to, he panicked and dismissed it because neither Hawkeye nor Potter did it (Hawkeye no doubt because he didn't like her, Potter perhaps only because they approached from behind him [if they did, I don't remember for sure]); which is to say, B.J. succumbed to a momentary bout of peer pressure, something one doesn't like to believe of one's role models. So, one of the things I'll go to my grave wondering is:

Were B.J.'s shorts really riding up?