Link to the original study: http://www.tandfonline.com/doi/abs/10.1080/00224490309552164
As far as to the sample they studied:
An initial sample of 248 male and female undergraduates at a regional campus of a Midwestern university participated as partial fulfillment of a research requirement in their Introductory Psychology course. To keep the sample somewhat homogenous, we used only data from unmarried, heterosexual, 18- to 25-year-old participants.
From The New York Times:
So how sketchy is the research on human sexual behavior, anyway?