My colleague Mark Steege & I co-authored 2 workbooks that are user friendly and address sexual issues. The older one is for care-givers of people with mental retardation. It addresses ways to protect and educate people with developmental disabilities. It helps the caregivers deal with any personal hang-ups that get in the way of their success as. 

The 2nd book is also co-written with Mark Steege and is more specific in that it addresses educating people with autism. It uses Social Stories to impart the information and also deals with what types of sexuality is developmentally typical. Both of these workbooks can be seen in more detail and can be purchased from Mark's website: www.steegepublications.com.