Does wearing tight pants or underwear like tighty-whities affect sperm count? - Carson*
Although some studies have shown a decrease in sperm count from wearing tight-fitting underwear, others have not.
The testicles may seem like they just get in the way sometimes. But the main reason "the boys" are on the outside of the body is because the body's internal temperature of 98.6° F is too hot for sperm production. So by "hanging out," the scrotum keeps sperm a few degrees cooler.
Too much heat can lower sperm count (but not enough to act as a form of birth control). So tight clothing and underwear that keep testicles closer to body heat in theory may affect sperm count. But many experts think there isn't enough of a temperature change to make any significant difference.