i have said before and will say again- I think that choice in underwear depends on what you're gonna be doing as much as anatomy. Guys with massive balls and monster dongs might need tight support, or they may need to have the freedom. Guys who are on the smaller side, or who expand and grow to the occasion might have more choice but still do usually have preferences... it's really all over the map. I think it would be fun to have reason to have a forced occasion to get 100 guys together and say... strip to your underwear NOW! and see how comfortable guys are with their choices, standing in a public space in just their base layer...
Also- I think that for most guys, once they are past the 'peer pressure' influences of living in the tight social packs of high school and college, will be more relaxed about the choice thing and which "underwear camp" they reside. Some days I want to wear boxers and feel like I have "arrived" as a breadwinner, like Mad Men. Other days, I want the feeling of a nice crisp CK boxerbrief- snug and sporty and athletic, like Marky Mark, or maybe Will Trueman.
Honestly, most days, it's tighty whites that I like- they seem to flatter my athletic frame and I can wear them "off the rack" and look youthful and hot.
To me it's not a question of whether it's briefs or boxers, but whether you're wearing brand labels that show a label, or you are wearing cheap brand/no name underwear, and whether you are wearing a belt and 3 layers of clothes that afford no peeks whatsoever, or a loose shirt that lets guys see you absentmindedly reach in and reveal a flat chesy amd ab set, and scratch/ readjust around other guys to show off a bit- either intentionally or just because...