A model to explain how college students come to engage in premarital sexual intercourse is estimated separately for men and women and by interpersonal commitment (engaged or going steady versus neither). The major finding is the large impact of friends' on students' own sexual behavior, both directly and as perceptions of friends' coital behavior mediate (1) religious values, and (2) more structural kinds of factors (off-campus residence, dating, fraternity / sorority membership).