This mixed methods study aimed to examine partner and relationship characteristics associated with HIV risk among young men who have sex with men (YMSM). A sub-sample of YMSM (18-25 years) who were involved in serious relationships with other men were recruited from two on-going longitudinal studies, Project Q2 and Crew450 (N = 20 couples). The mean age of the dyadic sample was 22.5 years (SD = 5.33, range 18-46 years) and participants were racially and ethnically diverse, with the largest percentage of the sample identifying as African American (47.5 %), followed by Hispanic (20 %). Participants completed individual self-report measures using computer-assisted self-interview technology and engaged in couples-based interviews. Mixed methods analyses indicated three global reasons for unprotected sex among YMSM in serious relationships: (1) the desire to achieve emotional intimacy; (2) the perception of being in a monogamous relationship; and (3) the difficulties associated with accessing and/or using condoms. Couples' decision-making processes, including decisions made "in the heat of the moment," have implications for HIV prevention interventions.