Or at least 237 reasons given by a survey of University of Texas students, as part of a research study conducted by Cindy M. Meston and David M. Buss (published in August’s issue of “Archives of Sexual Behavior”). I don’t have access to the full article via my university yet, but you can read a summary of the findings via the NY Times here.
The responses range from burning calories to making babies, but what surprises me is that those 237 reasons are only from about 400 students. Who would have thought there would be such diverse responses among such a small sample size? Quite amusing to say the least.