Young adults diagnosed with severe mental illness (SMI), intellectual disability (ID), and autism spectrum disorder (ASD) often experience significant barriers in accessing sexual health education, contributing to poor sexual and relational well-being. The Educational Intervention and Training Program on Sexuality and Affectivity (eITPoSA) was developed to address this gap through a structured psycho-educational approach tailored to their cognitive and emotional needs. A pilot study was conducted with 12 participants diagnosed with SMI, ID, and ASD, who attended the training sessions covering topics such as body knowledge, genital sexuality, privacy, and hygiene. Pre- and post-intervention assessments were carried out using structured monitoring grids, with data analyzed through descriptive statistics, paired-samples t-tests, and Cohen’s d to evaluate effect sizes. Additional insights were gained from the open-ended questions designed for Modules 1, 2, and 6, as well as from the qualitative analysis of the Circles in Module 5. Results demonstrated statistically significant improvements across all domains, particularly in body knowledge (31.43% to 86.65%), genital sexuality (15% to 90%), and hygiene (47.27% to 96.36%). These findings suggest that the eITPoSA program effectively enhances sexual and relational competencies in young adults with diagnosed with SMI, ID and ASD. Future research should focus on expanding the sample size and exploring long-term retention of educational outcomes to further validate the program’s efficacy.