This study examined specific circumstances precipitating onset sexual offenses in a sample of 278 Australian male youth who have sexually harmed. Offense narratives—a description of how the onset offense was initiated, interactions and behavior that took place leading up to, and during, the offense, and how the offense ended—were qualitatively coded to identify how the sexual offense incidents manifested, particularly the motivations that interacted with opportunities to engage in sexual offending behavior for the first time. Three overarching themes emerged: sexual curiosity and/or gratification in the context of usual activities, broader non-sexual offending behavior that resulted in an opportunistic encounter with the victim, and peer-related influences in the manifestation of sexual offending. The findings highlight the importance of ecosystemic and situational factors in the commission of youth-perpetrated sexual offending which could be addressed to prevent sexual violence.