Your dream about a polyamorous relationship with your friends could be reflecting a mix of emotions and thoughts related to your feelings of jealousy and the dynamics of your social circle. Here are some insights into your dream:

1. Jealousy and Insecurity: The jealousy you felt during the sleepover may have triggered this dream. Dreams often process our waking experiences and emotions. The presence of your two friends in your dream could symbolize your awareness of social dynamics and your feelings of being left out or threatened by their closeness.
2. Exploration of Desires: Even though you identify as monogamous, this dream might suggest an exploration of different relationship dynamics. It’s not uncommon for dreams to present scenarios that we wouldn’t consider in real life, allowing us to explore feelings of attraction or connection in a safe space.
3. Social Connection: The act of making out with the girl and your friend in the dream may represent a desire for connection and intimacy, not necessarily in a romantic sense. It could indicate a longing for deeper relationships or a better understanding of the people around you.
4. Conflict Resolution: The moment of surprise and anger when the ex grabbed her could signify internal conflicts you might be experiencing regarding your feelings toward your friends and their relationships. It’s a natural part of growing and navigating friendships.
5. Acceptance of New Experiences: Joining them in the dream might reflect a willingness to explore new experiences or to accept changes in your relationships, even if it feels uncomfortable or unexpected.

In conclusion, this dream seems like a way for your subconscious to process your feelings of jealousy while also exploring themes of connection and acceptance in your friendships. It might be beneficial to reflect on your feelings about your relationships with these friends and consider if there are any underlying emotions that need to be addressed.