Background: Despite a recent ideological shift towards the recognition of sexual autonomy for people with an intellectual disability (ID), there are continuing social and cultural barriers to sexual expression. Part I of the current two-part study assessed the sexual knowledge, experiences and aspirations of service users through focus groups and also examined their perceptions of impediments to achieving sexual autonomy. Method: Thirty-two participants (20 male, 12 female) attending an ID service participated in focus groups delineated by gender and age group (13-17 years; 18-30 years; 31+ years). Results: Analysis of the focus groups showed that service users, especially those over the age of 18 years, had an understanding of their sexual rights but also identified a number of social and cultural barriers that they felt prevent them from achieving sexual autonomy. Those under the age of 18 years had only rudimentary knowledge of sexuality issues, for example pregnancy and sexual anatomy, but aspired to relationships and marriage similar to those over the age of 18 years. Family and staff attitudes appeared to be very influential in the views of respondents. All service users had received some form of sex education, although the benefits of such education appeared most enduring for those over 18 years. Conclusion: Service users had an understanding of their sexual rights and the social and environmental barriers that prevent them from fulfilling their rights. The provision of sex education training and promotion of positive attitudes towards appropriate sexual expression is critical to the realization of sexual autonomy for people with an ID.