It’s well known that high testicular temperatures have a bad effect on the testicles and their sperm production (just like varicoceles and undescended testes). Saunas have been suggested to harm sperm production called spermatogenesis.
In a recent study from Italy of healthy men, reversible decreased sperm production and changed sperm DNA packaging occurred with use of sauna two-times per week at 80 or 90 degrees Celsius for each exposure. Six months were required after stopping of sauna use to restore sperm concentration to baseline. Men with occupations of elevated heat, even for short periods, should be aware that fertility may be at least transiently affected. If you’re trying to have kids currently or in the near future, keep the testes cool with a cool shower!