I think what makes tighty whities hot for me is that
a. I grew up in an era when ALL guys wore them. In the mid to later 70's any guy at my school who wore boxers was laughed out of the locker room for wearing grandpa underwear. Then, mysteriously, that all changed and the opposite situation dominated: anyone wearing tighties was laughed hot. I'll never understand it.
b. the sheer UNSELFCONSCIOUSNESS of guys wearing them made them hot. Guys are wear underwear that's too "precious" or which demands "look at me... II'm so sexy" are a turn off. I always think of some dude just grabbing whatever briefs were at hand, with no regard for how they "look" and just yanking them on (and later "yanking ON THEM!" ) and heading off to classes or whatever; that made them hot.I clearly recall a beefy football player whose jockeys were on inside out because he just didn't give a fuck. He formed part of my locker room fantasies for years.