"When you have sex with someone, you are having sex with everyone they have had sex with for the last ten years, and everyone they and their partners have had sex with for the last ten years."
          - C. Everett Koop, M.D., Former U.S. Surgeon General -

Here is a chart to explain that fact. Find the number of sexual partners encountered, and look across and you will find the total number of people exposed to, assuming they have had the same number of sexual partners as you have had.