Little research has examined the discourses that shape therapists' sense-making around heterosex. This paper explores the discourses of sexuality and gender underpinning therapists' and non-therapists' responses to a sexual experimentation scenario in a heterosexual relationship. It also considers the value of the novel technique of story completion (SC), in which participants are asked to write a story in response to a hypothetical scenario, for qualitative psychology and psychotherapy research. This research used a comparative SC design (Kitzinger & Powell, 1995). Participants were sequentially presented with and invited to complete two story stems: one in which a male character suggested 'trying something new' to his female partner and one in which the female character made the suggestion. The stems were otherwise identical. A total of 100 SCs were written by 49 (28 female; 21 male) therapists and 51 (29 female; 22 male) non-therapists. Participants were recruited mainly via UK-based email lists and Facebook groups, and therapeutic training organizations, and the data were analysed using a feminist post-structuralist thematic analysis (Braun & Clarke, 2006). Both groups of participants drew on heteronormative discourses of sexuality and gender to make sense of the stem. Engaging in sexual experimentation was often depicted as a demonstration of being normal. In some stories written by women, sex was framed as a site for negotiating 'equality' and reciprocity in relationships. Therapists were more likely than non-therapists to frame 'difficulties' within relationships as opportunities for personal growth and increased emotional depth, and their stories included greater emotional complexity. These findings raise questions about practitioner training and whether it results in therapists drawing on narrow and restrictive discourses of heterosex in clinical practice. Training on sexual issues is largely absent from non-specialist practitioner training courses, which potentially means therapists are ill-equipped to respond to clients' anxiety about sexual issues. Evidence from this and other research indicates that therapists' sense-making around heterosexual sexual relationships is underpinned by narrow and restrictive discourses that entrench traditional gender relations and limit sexual agency. Psychologists are increasingly taking up positions of clinical leadership and are looked to for models of best practice. Drawing on theorizations of sexual difficulties, and of anxieties about sexual practice, that challenge traditional gender and heteronorms, and the commodification and medicalization of sex, is important for effective psychological leadership relating to the treatment of sexual issues and the furthering of social justice agendas. © 2018 The British Psychological Society.