The need for comprehensive sexuality education (CSE) to be culturally relevant and inclusive is increasingly recognised as a fundamental aspect of supporting young people to live healthy sexual lives. Nevertheless questions remain about how to represent cultures and difference without subtly reinforcing inequalities. This paper makes a case for the need to explore this issue through analyses of how different gendered and demographic groups of young Tanzanian attendees of culturally relevant CSE, identify with (or against) intervention knowledge[s]. Grounded in dialogical social psychological theorising, we present a methodological approach for exploring how processes of belonging and Othering structure young people's negotiations of culturally relevant CSE amongst other knowledges. An adapted version of the 'story completion' method was used with university students and urban-poor young people (aged 18-34) to instigate dialogues about how a fictional protagonist might think, feel, and act in their relationship, looking to see if, and how, young people incorporated CSE knowledge. Twelve single-gendered focus-group discussions were held in September 2014 with 48 young people, and then findings from these were discussed further with 27 returning young people through three mixed-gendered workshops in August 2015. The analyses highlight how young Tanzanians explicitly Other CSE interventions, positioning their knowledge as 'not for us'. More implicitly, difference is also constructed around ideas about change and gendered development, along with trust and support in relationships. The devices used to Other shifted and differed across demographic groups, ranging from complete denials of intervention knowledge to viewing it as unrealistic, dangerous, or self-stigmatised for not being able to use it. We propose that these findings highlight the need to rethink how both 'culture' and 'relevance' are conceptualised in CSE, most specifically necessitating greater recognitions of poverty, transnationality, and the lasting legacies of colonialism and behaviour change interventions that communicated through fear and morality.