In males, the testes were very sensitive to pressure. A sharp blow to the organ could render males of Human and other species helpless.
In at least one species, the genitals were located in the right knee. This was different from many other species. This did not, however, affect the sensitivity of the organ. (Star Trek VI: The Undiscovered Country)
When Chief Miles O'Brien made an unsuccessful attempt to retrieve his time lost daughter Molly from Golana, he said "Bollocks," which was a colorful metaphor used to describe the testicles. (DS9: "Time's Orphan")