It was originally thought that wearing tight underwear could lead to infertility in men as it may raise testes temperature to a point where it interferes with sperm production.
In 1998 in The Journal of Urology, Drs. Robert Munkelwitz and Bruce R. Gilbert analyzed semen samples from 97 men with fertility problems. Half of them wore briefs, half wore boxers. The researchers measured the men's scrotal, internal and skin temperatures, both while the men were wearing underwear and again when they were going commando. They found no significant differences between the two groups of men in scrotal temperature, sperm count, sperm concentration or sperm motility.
The purpose of the scrotum is to maintain the testes at a temperature of approximately five degrees less than the rest of the body (about 93.6 degrees). It appears as though the scrotum does its work whether you are in boxers, briefs or nothing.
So you can make your own fashion choice and your sperm won't know the difference.