Background Concurrent relationships are a driver of generalised epidemics of sexually transmissible infections (STIs). In Australia, explicit negotiation of agreements about sex outside regular relationships has been recommended in health promotion for gay men but not for heterosexuals. Objective: To ascertain the annual incidence of concurrency among people in ongoing male–female relationships according to their reported expectations of exclusivity. Methods: In a national cohort recruited by household random digit dialling in 2004–05, people aged 16–64 years completed computer-assisted telephone interviews including questions about expectations of sexual exclusivity, discussion and agreements. A year later, those in ongoing sexual relationships (5323 people) were asked about sexual partner numbers in the past year. Results: The huge majority (96%) expected sexual exclusivity of themselves and their partner. However, only 48% of men and 64% of women had discussed the matter and explicitly agreed. Older respondents were less likely to report discussion. Only 1% reported mutually nonexclusive (‘open’) relationships. A year later, 93% of respondents were still in the same relationship, among whom 4% of men and 2% of women had had sex outside the relationship. Those with agreements that one or both partners could have sex with others were more likely to do so, but the majority of respondents who had sex with someone else were in relationships that were explicitly or implicitly expected to be exclusive. Conclusions: Sexual health promotion should stress the importance of STI testing and establishing agreements about exclusivity before condoms are abandoned in new relationships.