ABSTRACT The use of alcohol or illicit substances by youths may be related to sexual motives and experiences. Accordingly, child welfare workers need to address young people’s sexuality in their investigative work. In this study, we have examined the significance of different social categories, namely gender and sexual orientation, when child welfare workers assess young people with a substance use and their related sexual health. Fifteen child welfare workers were recruited through a strategic sampling to participate in semi-structured interviews, which were analysed using thematic content analysis. The results consist of three sub-themes: Sexually vulnerable girls; Boys as perpetrators; and Unseen LGBTQ+ youth. Together, these sub-themes constitute an overarching theme where the heterosexual matrix appears to be the interpretative framework when investigating a problematic substance use. The suggested implications for practice include education in sexual and reproductive health and rights for child welfare workers aiming to achieve an equitable assessment process for all youth with a problematic substance use.