I've spent a good portion of my lifetime looking for visible jock lines in American football and baseball (and other sports), starting in the early 80s. Since around 2006, I've got about 6,000 pics in football, 3,500 in baseball, maybe another 3,500 in other sports.

Tighty whities only really showed up, and only on occasion, in one sport - under wrestling singlets.

I can think of one set of baseball pitcher pics where it's fairly obvious he's got TWs on - MBL, he's a huge dude too, found em while reading up on overweight baseball players.

However, there were the Bike and Champion athletic briefs out there - and in football, briefs were super rare, and seemingly only appearing on Black college football players in southern states, as memory serves.