She might be going through something I went through when I realised exactly what me being poly means for my future. I was seeing this guy, he was so hot, so good in bed, so great, but this poly thing just wasn't him. At the time, I was also kind of seeing this other guy, a guy I knew would only ever be compatible for a secondary style relationship. Our long term goals were too different for us to like, live together or anything. I liked him though. And, more than anything else, it wouldn't have been ethical to just dump him. Needless to say, the first guy couldn't ever see me as relationship material, I was someone elses. That barrier he put up was so awful for me as well as the stark realisation that the majority of men are monogamous. It was quite a time of mixed emotions. Yes, I did feel a element of resentment towards the other guy I was seeing for a bit.
I saw this guy a few more times after I knew the score. Just did. If I'm totally honest I text him in a low moment recently. No reply. I don't know why because I know even if we feel madly in love, monogamy wouldn't make me happy. Just stupid shit that people do, I guess.