No, it doesn't mean you want to get back together. If you have a racy dream about your ex, don't pick up the phone. It most likely means that you're still processing some unfinished feelings about him or her. If you are with someone new, the ex sex dream might signify fears you are having about your new relationship and/or the resurgence of the types of feelings you felt with your ex. Pay close attention to the nature of the sexual experience and how you felt during the dream, as it will give you insight into what type of unfinished feelings you have about your ex.
For years and years, I had a variation on a sex dream with an ex where I would be in bed with him and find girls hiding in his bedroom or under his bed and I would feel betrayed. Although he never cheated on me, I came to understand that these dreams represented me always feeling like he was hiding something from me and I never really understood why things ended between us.