In this paper, we describe a user-centred design process, where we engaged with 58 adolescents over an 18-month period to design and evaluate a multiplayer mobile game which prompts peer-led interactions around sex and sexuality. Engagement with our design process, and response to our game, has been enthusiastic, highlighting the rich opportunities for HCI to contribute constructively to how HCI may contribute to sexual health in adolescents. Based on our experiences we discuss three lessons learnt: lightweight digital approaches can be extremely successful at facilitating talk among young people about sex; sharing control of the conversation between all stakeholders is a fair and achievable approach; even problematic interactions can be opportunities to talk about sex