Heat and Testicles

In humans, as in most mammals, spermatogenesis is temperature dependent. This temperature dependence has been clearly demonstrated by several experimental studies showing that artificial increases in scrotum or testicle temperature in fertile men reduce both sperm output and quality. study 1998

Higher temperatures lead to an increase of testicular metabolism that results in spermatic damage. Oxidative stress is the main factor responsible for testicular damage caused by heat stress. The aim of this study was to evaluate lasting effects of heat stress on ejaculated sperm and immediate or long-term effects of heat stress on epididymal sperm. We observed decrease in motility and mass motility of ejaculated sperm, as well as an increase in the percentages of sperm showing major and minor defects, damaged plasma and acrosome membranes, and a decrease in the percentage of sperm with high mitochondrial membrane potential in the treated group until one spermatic cycle. - study 2016

There is literally a device that uses deep heat to cause temporary infertility.

Here is an article that talks about relationship of testicle heat and Testosterone. However, its not as conclusive as sperm decrease due to heat exposure.

Here is a sauna related study:

The effect of a single sauna exposure on spermatozoa: Semen was collected at weekly intervals for 3 wk before and 10 wk after the sauna exposure at 85 degrees C for 20 minutes. The numbers, morphology, ultrastructure, motility, viability and metabolism of the sperm was assessed. Sperm numbers fell within one wk and slowly returned to normal in 5 wk. - study