From BBC NEWS:
Men who use laptop computers could be unwittingly damaging their fertility, experts believe.
Balancing it on the lap increases the temperature of the scrotum which is known to have a negative effect on sperm production, researchers found.
…Just sitting with the thighs together, a posture needed to balance a laptop, caused scrotal temperatures to rise by 2.1C.
When the men used a laptop in this position the average temperatures increased by 2.6C on the left of the scrotum and 2.8C on the right.
Article continues with more information than I needed.