I suggest it might be generational, with a cyclical factor thrown in. Showing my age here, but when I entered Grade 7 (11-12 year old classmates) phys ed, and had to change / shower for the first time, absolutely NO ONE wore boxers. they were considered the kind of underwear your grandfather would wear, and no one else. EVERYONE in the changeroom wore briefs, usually white standard briefs, typically bought by your mother, and usually a size or two so you could "grow into them". Almost unbearably ugly, loosefitting, and if you had reached the start of puberty, damned annoying, because the material allowed your equipment to flop all over the place. About two weeks into the year, a new guy arrived at our school from Europe, wearing brightly coloured microbikini briefs. Showed off his body, looked great, masculine and HOT, and well, that was the end of the plain white briefs for most of the guys in my phys ed class. Looking back, it was almost like there was an unofficial contest to find the tightest, tinyest bikini briefs you could stuff yourself into. Then came the BULGE aspect, with each guy carefully arranging his equipment to show the most obvious bulge, with nothing left to the imagination. I was fascinated by all of this, because it didn't seem to matter if the bulge showed a small cock and nonexistent testicles, or a good sized cock and balls (for those further along in puberty). The goal was to be as obvious, and as large as possible, which carried over into the skintight jeans that were in fashion at the time. Most of us would be stuffing semi's into our bikinis, stretching the elastics all to hell. And this was back before guys got so selfconscious ... there were plenty of comments and compliments, often asking where the wearer got those "really cool bikinis". Tighter the better, tinier the better. my memories of this time are etched into my skull. this continue throughout middle and high school into university days, when you could see every body type wearing the most miniscule bikinis imaginable. Nowadays, its boxers or boxerbriefs, usually worn extremely low on the hips, to showoff the fact that the wearer trims or shaves his pubes.
