Human immunodeficiency virus (HIV) is a global health problem, with sexual risk behaviors (SRB) being the main routes of spreading the virus. Evidence indicates that different psychological factors influence SRB (e.g., attitude towards condoms, sexual self-concept, sexual sensation seeking, knowledge of sexual risk behaviors, risk perception). This study proposes an explanatory model of sexual risk behaviors in young people and adults. The sample consisted of 992 young people and adults aged between 18 and 35 years. The model presented good levels of fit (X2 = 3311.433, df = 1471, CFI = 0.964, TLI = 0.959, RMSEA = 0.036), explaining 56% of the variance of sexual activity with multiple partners, 77% of the inadequate use of protective barriers, and 58.8% of sexual activity under the influence of alcohol or drugs from a set of psychological factors in self-report measures. The details of the results offer novel contextual evidence for the prioritization of prevention-oriented psychosocial interventions.