There's a plausible theory that the testes are carried outside the body not to protect them from the elevated core body temperature but instead to protect them from elevated pressures in the abdominal cavity caused by vigorous movement (kind of like when you tense your muscles to defecate if you'll pardon me saying so). The temperature-sensitivity would result from having evolved for optimal sperm production at the lowered external temperature. I don't know details of this theory as it was just mentioned in passing by a colleague.