This study was designed to explore Taiwanese school students’ attitudes toward sexual relationships and premarital sex. This was an exploratory descriptive, qualitative study. Focus groups (N = 8) were conducted with 47 adolescents from three high schools in Taiwan. Transcripts were transcribed and thematically analyzed using Atlas V 5.0. Adolescent attitudes toward sexual relationships and premarital sexual behavior comprise the following three dimensions: (1) external incentives, (2) the developmental process, and (3) internal control. External incentives include the normalization of sexual behavior between peers, the desire to feel included in a group, parental influence, and media influence. The developmental process includes imagining the sexual experience and onset of sexual activity. Internal control includes the fear of pregnancy, the fear of parental rejection, and the fear of being judged. These findings can provide a reference for designing future sex education curricula and counseling programs for adolescents.