Modern trousers are usually too tight and dense, and become much tighter when you sit down. Clothing designers have not been paying attention to the needs of male anatomy and physiology. The penis and testicles have obviously been designed to be free and move with the rhythm of walking. This is clearly important to the health.
Both trousers and underwear have a seam in the middle which gets even tighter when you sit down. This disturbs the nerve functions and the sensitive but vital circulation of the genitalia. To prevent this, men should use very loose underwear, such as boxer shorts, and loose pants.
One cause of deterioration of the sperm is the rise of the temperature of the testicles up to 37°C when you use ordinary underwear and trousers. The optimal physiological temperature for the testes is 33°C. The testes have 4 separate mechanisms trying to keep this normal and healthy temperature. Normal clothing and sitting habits eliminate them all.