Sperm need to be kept at a lower temperature than what the human body is. If the testicles were inside of a man's body, the sperm would overheat, leading to defects in the sperm and consequently rendering the male infertile. Interestingly for a period when tight jeans were fashionable for men, many men were reporting infertility, which was discovered to be due to their tight jeans holding their testicles too close to their body and therefore resulting in the overheating of their sperm.
