Well, i'd say there is most likely a drop in sperm viability as heat goes up in the factory.
Now, this will depend on the depends used and what else. If you are wearing a diaper snug up and maybe a snug onesie or pajamas it would make it worse.
But, overall, i'd say it seems to be secondary for the most part. Like it will lower your count, but if othewise things are normal you'd be fine.
IIRC we are talking here in the millions of little swimmers, and if say the diapers lowered it by 10% still going to be in the millions. Now yes if your count is low, it may be something you can do to raise the count.
But there are LOTS of OC people that have kids, not to discount any issues, but IMHO diapers would not be the #1 cause to having low sperm count.
Now sitting for hours every day in a hot tub at 104deg is shown to have an effect, but the hotest you should be in a diaper would be 98.6deg F. Else there are other things that might be a way bigger issue.
So, if your sperm count is low, yes cooling the chaps would maybe help, but what else is causeing the low sperm count?
As is the answer, usually this will take a doctor and tests to figure out what exactly is going on, but do tell them everything inclusing the diaper use, never keep things from your doctor, not worth it.'