Zambian Demographic and Health Survey data reveal that increased discordance between professed attitudes and measures of behaviour regarding premarital sex among adults is strongly associated with increased risk of HIV in adolescents, particularly girls. We hypothesised that this was due to the reluctance to talk about premarital sex, resulting in a situation we call the "taboo gap" where sexual behaviour is a forbidden topic and adolescents feel unable to seek advice or sexual and reproductive health services. Our analysis revealed that the taboo gap is rooted in harmful gender norms that are perpetuated by schools, churches, cultural influences, development programmes and health systems. Challenges like food insecurity and household poverty may place girls in positions where they are vulnerable to sexual exploitation, increasing their risk of exposure to HIV. Unmarried adolescents, particularly girls, report being ridiculed when they go to reproductive health clinics, which discourages them from seeking care in the future. Strengthening peer support and parent-child interactions are important programmatic elements. We conclude that discordance serves as a novel measure and harbinger for the presence of gender norms which generated a taboo gap that impeded carseeking and increased risk for HIV among adolescents, especially girls, in Zambia. We propose that successful interventions must involve a multifaceted, gender transformative approach which engages peers and stakeholders in schools, churches, clinics, and families, particularly parents, to reduce the gendered gap in HIV risk and transmission.