Tight boxers generally do not harm your health, but according to a Harvard study, they can still reduce the number of healthy, swimming sperm. The total number of swimmers was not only 17 percent higher among those who wore wide boxers, but the sperm cells were 33 percent fitter. This is related to body temperature. The optimal testicular temperature is around 33° Celsius, which is lower than human body temperature. Free-swinging testicles feel more comfortable than those that are pressed against the body using tight boxers or jeans.