This study examined how people define having sex utilizing a new approach to this area of research. A total of 267 men and 327 women rated their degree of confidence that engaging in each of 21 physically intimate behaviors (e.g., penile-vaginal intercourse) counted as “having sex” and then qualitatively explained their reasoning. Separate ratings were made for each behavior when engaged in by the respondent and by his or her partner with someone else. Results showed that, as in previous studies, for both sexes, some behaviors (e.g., penile-vaginal intercourse) were far more confidently rated (i.e., “definitely sex”) than were others (e.g., oral-genital stimulation). Further, both men and women were significantly more certain that a behavior counted as “having sex” when considering their partner's behavior outside the relationship than when they considered their own behavior. Finally, the order in which the two scenarios (i.e., self versus partner) was presented significantly affected participants’ certainty. Qualitative results, paired with quantitative findings, suggest that individuals consider a variety of contextual factors when making these definitional decisions. The methodological and sexual health implications of these results are discussed.