From THIS is TRUE for 14 July 2002:

Sex and Lifespan

Researchers at England's University of Sheffield say having more
sex may shorten your life. "Those organisms that mate the most, and are
therefore more successful in evolutionary terms, reduce their own life
expectancy in the process," says Dr. Michael Siva-Jothy. He says mating
releases a hormone that damages the immune system. While they studied
mealworm beetles, they say the findings could hold true for humans,
too. (Reuters)