I think your dream is telling you that you have not forgiven your ex.  Even though you are no longer together, he is still with you, in the sense that you are holding on to your grudge.  In your dream, he appears as a symbol of your relationship with him.  He is asking you to have sex with him, meaning that you need to find wholeness and restoration in your relationship, even though you are not together anymore.  Does that make any sense?  I'll try to elaborate more, if that's not clear.  Let me know.

You are refusing him, meaning that you are not open to the idea of forgiveness.  You'd rather hold a grudge, if I may be so bold as to say so.  And no wonder.  Forgiveness is not easy, especially when the person doesn't deserve to be forgiven!  But you will never become a whole person or healthy if you don't find release from this.

The fact that this dream is recurring means that your subconscious is telling you that this issue is vital to your emotional and spiritual well-being.