Background: Young adults diagnosed with severe mental illness (SMI), intellectual disability (ID), and developmental disability (DD) often face challenges in accessing comprehensive sexual health education. The educational Intervention and Training Program on Sexuality and Affectivity (eITPoSA) aims to address this gap by providing tailored psycho-educational interventions. Methods: A pilot study was conducted with 15 participants diagnosed with SMI, ID, and DD. The program comprised six sessions focusing on body knowledge, genital sexuality, privacy awareness, privacy protection, and hygiene and personal care. Pre- and post-intervention evaluations were carried out using monitoring and evaluation grids. Data were analyzed using descriptive statistics, paired-samples t-tests with bootstrapped confidence intervals, and Cohen’s d to assess effect sizes. Additional valuable 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: The program demonstrated significant improvements in all areas evaluated. Mean scores for body knowledge increased from 31.43% (SD = 25.18) to 86.65% (SD = 23.98), genital sexuality from 15% (SD = 34.81) to 90% (SD = 24.00), privacy awareness from 82.94% (SD = 25.10) to 98.75% (SD = 7.16), privacy protection from 86.11% (SD = 29.65) to 95.95% (SD = 14.45), and hygiene and personal care from 47.27% (SD = 38.23) to 96.36% (SD = 12.06). All improvements were statistically significant with large effect sizes for body knowledge, genital sexuality, and hygiene and personal care. Conclusion: The eITPoSA program significantly enhances sexual and relational knowledge among young adults with SMI, ID, and DD. These results support the efficacy of tailored sexual health education programs. However, further research with larger, more diverse samples is needed to generalize these findings.