It pops up again and again in this thread, but I genuinely don't understand this, "It's okay for me to fuck around on you but don't you dare so much as look at another person" mindset. On one level, I get that poly people are almost without exception selfish assholes with little to no self control, but on a more visceral level it's hard to grasp.
Gee, you don't like that your main squeeze is hanging out with/interested in/fucking another person? But you don't think they have a right to feel bad when you do it to them? Poor baby, that must be so difficult for you.
It's just salt in the wound that the go-to response to your partner expressing discomfort in this setup is, "They're trying to control me and limit my happiness!" No, they want an intimate bond with another person who makes them feel irreplaceable and deeply loved. That's hard to do when you have made it very clear to them that you are happy to get dick/pussy on the side and will pout when your significant other has to go and be a human being with inconvenient feelings and insecurities of their own.