Just a remark: Your balls don't need the warmth of your body to produce sperm; it is exactly the other way around: They need to be a few degree under your body temperature to produce sperm.
Yes, finally, i know. But just a few degrees. That's why the scrotum is usually close to your body and only gets longer a bit when it's really warm and the balls don't need the body heat to keep their working temperature ;-). But if you stretch them long enough, there will be no way for them to get close enough to that heating system when it is actually too cold. That's my theory at least ;-)