The primary aim of this qualitative study was to explore the influence of HIV treatment optimism on the sexual behavior of people living with HIV/AIDS (PLWHA) receiving highly active anti-retroviral therapy (HAART) at public health clinics in Rio de Janeiro, Brazil. We also explored the psycho-social dynamics of participants' sexual relationships in order to understand more broadly how these factors influence the sexual behavior of PLWHA and how they shape HAART-related beliefs. Twenty-three semi-structured, in-depth interviews were conducted with three groups: heterosexual women, heterosexual men and men who have sex with men living with HIV who reported being sexually active in the last year and were currently receiving HAART. We found that the availability of HAART was conceptualized as a rationale for unsafe sex among a minority of study participants and that this was more common among men than among women. Specific examples of treatment optimism appeared to be employed as a means to avoid acknowledging how deeper psychosocial issues may have been influencing participant's sexual behavior. Most participants' sexual behavior appeared largely to be a product of their desire for social validation and linked to feelings of shame and denial, including but not limited to HIV. Participants from all three groups expressed a considerable amount of fear and/or anxiety regarding behaviors such as disclosure and condom use as a result of the unexplored conflict between implementing these behaviors and continuing with their strategies for social validation within the context of their sexual relationships. We conclude that short-term information, education and communication interventions surrounding treatment optimism, disclosure and condom use are appropriate and necessary; but that they are not sufficient to address the core challenges to unsafe sex among PLHWA. These deep-rooted psychosocial issues may be better addressed by longer-term individual and group-level opportunities for exploration and critical reflection regarding sense of self and its relationship to social solidarity among PLWHA.