Using an extended definition of sexuality, this mixed-methods study builds on existing research into adolescents' emergent sexual development by longitudinally examining adolescents' sexual behavior trajectories (i.e., from less to more intimate sexual behavior). Over a 2-year period, 45 adolescents (M age = 15.9 years) reported on their sexual behavior using questionnaires and on their everyday expressions of sexuality in the form of semi-structured diaries. Cluster analysis using the questionnaire data identified three sexual behavior trajectories: a non-sexually active trajectory (meaning no or minor sexual behavior) (n = 29), a gradually sexually active trajectory (meaning step-by-step sexual behavior development) (n = 12), and a fast sexually active trajectory (meaning rapid sexual behavior development) (n = 4). Qualitative analysis using diaries revealed the following themes: romantic versus sex-related topics, desires, uncertainties, and references to the social context. In general, all adolescents reported more about romantic aspects of sexuality (than about sexual acts) in the diaries, regardless of their sexual behavior trajectory. Sexually active adolescents (i.e., gradual and fast) were more concerned with sexuality in their diaries, especially more with the physical aspects of sexuality, than non-active adolescents. Gradual adolescents experienced more desires about physical sexual contact and reported fewer references to their social network than non-active and fast adolescents. The findings suggest that sexual education that discusses the internal experiences of sexuality, such as feelings and thoughts, particularly the romantic aspects, may help adolescents process their preferences for different sexual and romantic acts and may contribute to healthy sexual development.