Despite the advances in the field of mental health resulting from the psychiatric reform and high HIV infection rates among users of mental health services, the lack of STD/AIDS prevention policies for this population, both in Brazil and abroad, is evident. This gap is exacerbated by the difficulty mental health professionals have to deal with mental health service users’ sexual activity, a fact that is supported by the literature and my professional practice as a nurse and researcher in the area of mental health. Based on a qualitative approach, this study analyzes the perceptions and practices related to mental health users’ sexual activity and STD/AIDS prevention of mental health professionals working for three types of services of the Psychosocial Care Network (RAPS) in the municipality of Rio de Janeiro (Rio de Janeiro state, Brazil). From June 2015 to February 2016, eight focus groups were conducted with teams of mental health professional from: a public university institution, a type III psychosocial care center (CAPS III), and an outreach project for the homeless (“Consultório na Rua”). Most of the 61 practitioners (40 female and 21 male) in the focus groups were between 20 to 40 years of age, and there were residents and professionals of several areas (including psychology, nursing, medicine, occupational therapy and social services). The professionals said they were aware users of mental health services are more vulnerable to violence-related issues, including sexual violence. However, sexuality is only approached in the scope of healthcare when a mental health user requests it or if a sexual violence incident occurs. All practitioners said they recognize the importance of approaching sexuality and STD/AIDS prevention issues, but we found that they rarely do this when caring for users of mental health services and that they do not go to team meetings to participate in the discussions. Moreover, those themes are rarely approached during their daily healthcare routine due to a lack of training and knowledge and because the institutions do not provide guidelines and policies. The findings attest to the need and importance of incorporating the topic of mental health users’ sexual activity into the education and training of mental health professionals of different fields in order to avoid sexual abuse and HIV infection. Our conclusion is that, despite the advances brought by the psychiatric reform worldwide and in Brazil in terms of deinstitutionalization and more comprehensive and humanized healthcare, sexuality, sexual health and STD/AIDS prevention have not been incorporated into the debate in the new provisions focused on social inclusion and the recovery of mental health users’ sense of citizenship.