During the transition to adulthood, many young people move away from home, begin careers, and engage in intimate relationships that may become sexual. While previous research has focused on individual-level risk factors of sexual debut, little is known about how the characteristics of intimate relationships are associated with the onset of first sexual intercourse within specific relationships. We use data from the Relationship Dynamics and Social Life study, which is based on a random sample of 1000 18-19 year old women in a Michigan county who were followed weekly for 2.5 years. We estimate hazard models to investigate the onset of first sexual intercourse within relationships. We find that conflict and power imbalance hasten the pace of sexual intercourse – relationships that include fighting, non-monogamous sex, and age and/or race differences between partners become sexually active at a faster rate, highlighting the importance of power and coercion within young people’s relationships.