The concern with tight underwear in men is all about heat. Sperm cells are sensitive to rises in temperature. In fact, they are so sensitive to heat that your own body heat is a problem. The testicles are held loosely by the scrotum to keep sperm cells further away from a source of heat, your own core temperature. In order for testes to produce sufficient quality and quantity of sperm, the temperature of testes must be lower than the core body temperature.