The whole point of a scrotum is to keep the testicles a few degrees cooler than the body's internal temperature. Too cold and the scrotum tightens. Too warm and it loosens.
Briefs can be a bit tight, preventing the scrotum from getting the balls farther from the body. OTOH, there's not a lot of air flow in there anyway with clothes, unless you wear something like a kilt, no? Are you a Scot?
