My boyfriend and I were in a part-poly relationship where we’d see people together (both male and female) only he went behind my back and cheated on me several times with another woman. He still wants her in his life and me, yet I don’t want to be anywhere that she is and don’t really want to be in a relationship that she is. She is also in a ‘poly’ relationship - I put in inverted commas because her partner isn’t in the know about anything. What advice can you offer?
Well this is an easy one. Leave this relationship, friend! Your partner violated the terms of your relationship, is now pushing you to do something you absolutely do not want to do, and wants to involve you in a very unhealthy relationship dynamic with the woman he cheated on you with who is also cheating on her other partner.
Sure, there's a small chance that you can resolve this with the all-mighty power of communication. You could sit your partner down and explain how hurt you were that he went behind your back and did something that the two of you did not agree to, and that the thing he's asking you to do is unhealthy and unreasonable. And there's a chance that he goes "oh, my goodness, you're right, I should not have cheated and I should not be pushing you to get involved in a deeply unhealthy situation, and I will stop that immediately, and begin therapy and self-work to understand what assumptions and entitlements led me to make these bad choices so I can heal this relationship."
But honestly, that seems very unlikely and probably not worth the extensive emotional effort it would take on your part. So my advice is to walk away. This guy is not good at respecting your feelings and boundaries in this relationship, so stop being in the relationship.