If you were more daring, you skipped underwear and flew commando. Sometimes you could see a few stray pubes peeking out of the buttoned fly; this was before anyone even thought about trimming their fuzz down there. I didn't fly commando until probably my third year of college. I wore 501s virtually every day through the whole four years.
There's nothing quite like wearing a pair of 501s commando when the other bloke hurriedly unbuttons you to get at your stiffening goods. If you're just getting a quick blow-and-go somewhere, you might only have to unbutton the bottom few buttons.
When I gained weight, I stopped wearing 501s and segued into "relaxed" jeans. Underwear is necessary with those because your junk moves around and you can get a nasty rub or pinch. When I lost weight again, one way I celebrated was by buying some 501s -- the traditional kind that had never been washed and needed to "shrink to fit." During the shrinking phase, you pretty much have to wear underwear because they're so rough. Now I'm back to my soft, worn 501s most every day and flying commando once more.
Anthony here is wearing 501s but they're not the shrink-to-fit variety. He's wearing the ready-fit, pre-washed variety. I'm that much of a 501 connoisseur I can spot the difference in a picture like this.