Excessive heat affects the sperm count
A good read recommended by Dr. Elist:
Excessive heat has been shown to have adverse affects on sperm count.
A man who works in an excessively hot climate or has a job that forces him to be around excessive heat or spends a lot of time in a hot tub, sauna or hot bath for long periods, all could affect his sperm in a negative way.
There has recently been some research done that suggests that men who spend a lot of time with their laptops or mobile phones in their laps can be at risk of overheating their sperm and damaging their fertility as well,
Other factors such as heating blankets and water beds,or tight biker shorts and wrestling outfits can constrict the testicles causing too much heat to build up, potentially damaging the sperm.