Tight clothes can affect you reproductive system in dramatic way, the reason behind is that tight closes will raise the temperature of the testes which has to be lesser than 98º Fahrenheit (which is the temperature of the internal of male human body) which will lead to dramatic decrease in sperm production and viability, that’s why man tests hanging outside the pelvis the way they are.

Cooler environment is required for healthy sperm population and sperm viability.
That’s happens in 15-25% of average age men without been noticed or   diagnosed with ‘sperm count” test which mostly not be done unless there is a problem in pregnancy , which may be too late stage to recover the condition and restore the normal healthy sperm population, so it is advised for every male in the reproductive age to do this test at least once every 3 years and  before long term relations .

Tight clothes may also cause a clinical case called (Testicular Varicocele) which characterized by painful swelling of one or both of the testes due to pressure being exerted on the vein drainage of the testes which leads to enlargement of the vein near the testes and stocking of the blood in the dilated vein which forms that (Varicocele), (Testicular Varicocele) affects the reproductive ability and sexual fitness together and in 60% of the cases, surgical involvement is a must.