Ehh, I don't know. I'd like to think balls are a little more hardy than that. Sensitive, sure, but not that sensitive. But then I don't have any, so I could be wrong. I've heard the same shit for women too, up to and including how heat can cause miscarriage, which is a load of shit because if it were true, no one would ever get abortions - they'd just soak in the tub for a while if they didn't want to be pregnant.
I mean if sperm, eggs, and fetuses can survive damn near 99 degrees inside our bodies, I think they're probably slightly immune to heat and won't spoil like milk in the sun. Now you pour some molten lava on a guy's junk and that'll render him infertile... and quite possibly dead too. But still infertile.
Besides, if hot temperatures caused infertility, there'd be no population growth near the equator. In any case, I don't think fertility is what people should be worrying about when it comes to climate change. More people is what's fucking shit up, we don't need to be losing sleep over how the fucked up climate might possibly impact our ability to make more people.