With the increased use of data-driven algorithmic curation of social media content and online advertising, young people are increasingly likely to come across content related to sex, sexuality and relationships that they have not searched for (i.e. non-intentional or incidental exposure). This study sought to explore incidental exposure to content about sex, sexuality and relationships among young people in Australia and considers both the applications and implications for sex education. Semi-structured interviews were conducted with young people (N = 22) aged between 18–25 years, and findings analysed by thematic analysis. Incidental exposure was often a consequence of either routine social media use, or intentional instances of information seeking. Participants were largely receptive to information about sex education encountered through incidental exposure. Incidental exposure appeared to be helpful for: (i) highlighting gaps in participants’ knowledge; (i) revisiting and updating knowledge about sex and relationships; (iii) providing opportunities to reflect on these topics; and (iv) understanding these topics from the viewpoint of individuals of other genders and sexualities. Incidental exposure was a prominent dimension of young peoples’ engagement with online content that should be leveraged and planned around in future digital health and educational interventions.