The general problem is that sperm production is most efficient at about 1 degree less than normal body temperature. Thats why the testicles are out side the body in a sack.
Therefore any added temperature added to the testicles makes them work at less than optimum. For this to be a problem the laptop needs to be in close proximity to the body i.e. directly on the lap, if it was on a desk for example this wouldn't be a problem.
So it's not really a case of causing damage but just stopping optimum function. Over the long term there may be adverse effects but small exposure is unlikely to be much of a problem as far as I know.