Cotton kills. You are all sketchy. There are countless cases of testicular hypothermia documented in ANAM, isn't that enough to convince you?
Every piece of synthetic fiber that comes into contact with my ass retains a whiff of that lovely aroma forever. Heat and exertion only cause the dormant stank to reassert itself with greater fury each time. Perhaps silver-lined shorts, as ketch suggests, are the cure, but I'm sure they're not cheap.
Cotton and wool, on the other hand, come out of the wash fresh as a daisy and stay mellow for a while. I've never had a problem with comfort with natural fabric undies, any season of the year. Except that one time I skied down St. Helens in my underwear and fell a lot.
Cotton briefs with those "nice flat seams" are definitely available.