Much attention has been focused on efforts to reduce unintended pregnancy by improving contraceptive use among high-risk women; however, there is limited information to guide interventions to engage young men in contraceptive decision-making. We conducted focus groups of young men, aged 19-26, from diverse racial backgrounds from low-income communities in the San Francisco Bay Area to examine social norms about sexual relationships and how they impact on contraceptive use. The data were analysed using content analysis. A range of relationships were described, however casual relationships predominated. While young men expressed strong desires to avoid pregnancy in casual relationships, the unpredictable nature of relationships, together with low communication and regard for the women involved, made stressing consistent contraceptive use among partners unlikely. The themes expressed by these young men about sex and behaviour in different relationships illustrate a spectrum of decision-making dilemmas and illustrate the inherent difficulty in fully engaging young men in contraceptive decision-making. A strategy is needed to address relationship values, dynamics and condom use beyond STI-prevention frameworks and young women's ability to make appropriate contraceptive choices in light of the inherent difficulties and uncertainty associated with casual relationships.