why do I have reoccurring  dreams about sex with my ex boyfriend ?

Because you miss him!  He is an ex boyfriend for a reason but that doesn't stop the very human feelings and sexual desires you had for him.  So it sounds as if it's time to move on and find a decent guy to date, and maybe build a Life with.   

The dream might be telling you not to abandon what you are familiar with when you gain new knowledge - I believe the ex boyfriend symbolizes old habits like brushing your teeth or cooking healthy meals or socializing with friends from child hood - these old habits should not be totally ignored when you incorporate new changes into your life.