This article presents the results of a research about the implementation of sex education in the public schools of Novo Hamburgo, Rio Grande do Sul, Brazil. Quantitative and qualitative methods were used and semi-structured interviews were conducted with 124 teachers from 56 public schools. The Discourse of the Collective Subject method was applied to analyse the interviews. The results show that sex education is not a crosscutting theme in schools, and that it is mostly dominated by the biomedical discourse. This means that non-heterosexual practices and identities are neglected and that representations of gender and sexuality that reproduce and legitimate gender disparity are accentuated.