A study at the Cleveland Clinic showed cell phone handsets in a trouser pocket could decrease sperm motility. Specifically, carrying a cell phone in the pocket while talking on a Bluetooth device was studied. There was a significant decline in semen quality, possibly from the phones high-power radiofrequency electromagnetic waves emitted while in "talk" mode. While more studies are needed, several other recent studies have supported these findings.

Source:  Renal and Urology News, Dec. 2009