There's a link between increased temperature of the scrotum and reduced semen quality, but it's uncertain whether wearing loose-fitting underwear improves fertility.
Testicles
The testicles produce and store sperm. If they're damaged, it can seriously affect the quality of your semen.
This can happen as a result of:
An infection of your testicles
Testicular cancer
Testicular surgery
A problem with your testicles you were born with (a congenital defect)
When one or both testicles hasn't descended into the scrotum, the loose sac of skin that contains your testicles (undescended testicles)
Injury to your testicles
Sterilisation
Some men choose to have a vasectomy if they don't want children or any more children.