I was just thinking that as sorry as I am this is happening, I'm glad you're posting. It makes me think that if the thought pops into my head to encourage my mono bf to try poly, maybe I should refrain. I'm not saying Sun's behavior is your fault -- it's not, he is responsible for his actions -- just speculating.
Maybe mono people can't fall into openly and happily loving more than one person like poly people can. Like, maybe Sun can't bring himself to say to Barbie "I love everything about you AND I love everything about my wife" because he himself is too uncomfortable with that fact. My impression is that for most hardcore mono people (as opposed to monoflexible peeps) the very idea of someone loving multiple people is extremely difficult to process and understand, which is why they often feel so betrayed by poly partners, so it makes sense to think that they would have a hard time dealing with it in themselves too.