Because you think your current boyfriend is not doing as well as his ex-boyfriend in some places or sensitive places. You miss your ex-boyfriend, but in real life, you refrain from thinking about him, or imply that you two are finished. Will cause you to have such a dream. The data you provided is not comprehensive, so I can only analyze it for you through my subconscious.
When you dream, you are in a deep sleep state. At this time, your brain has lost most of its thinking ability, and the rest of the work will turn your subconscious into an image and show it to you. The subconscious is the truest portrayal of your heart, because what you really think at this time will not be limited by your thinking, and you still have your ex-boyfriend in your heart. I like dreaming very much, because analyzing the subconscious is interesting and real. Because too many times we are deceiving ourselves.