My first thought about why it took her so long to tell you about this, was this: It's been built upon in her mind after the fact. While it may be true that R was uncomfortable at the meeting, I wonder if she then related the experience to one or more close friends who are also mono (I don't know what "monogamish" means, really) and who wouldn't have looked at it from a poly perspective.
Without context, it could easily be viewed by someone who doesn't know anything about ethical non-monogamy as inconsiderate, insensitive, and rude for you to have any interaction at all with someone else in front of her. "He did what?!!! Made you sit through that?!! They looked at each other?!! He kissed her?!!! OMG what a cad, you poor thing!"
So maybe some outside opinions from people have poisoned her thoughts about it. She had some discomfort and others' reactions fed into it. We humans are very easily influenced by others, especially when we feel vulnerable, so eventually you hear an opinion over and over again until you think it is your own, and then we start believing it and making it bigger and bigger. If I were you, I'd ask her if she talked to any mono friends about it and, if so, did they offer opinions on the meeting. Start to see if she can deconstruct where the discomfort and upset came from.
Last edited by nycindie; 04-07-2013 at 12:29 AM.