It didn't upset her. I think you have totally misread that scene.
It was played for laughs.
This.
Too, it was also all during their little escapade trying to retrieve their memories, and during which Riker and Troi still on a subcoscnious level recognized that they two shared a very special connection, that might have led to them rekindling their romance had Ro not been there too. It's not so much that it all happened "under Troi's nose" as it was that Troi's not stupid. She knows Riker was dillydallying with her and Ro at the same time and now that he'd bedded Ro, the same was not going to happen (as easily, if at all) with her. It wasn't like she felt betrayed or slighted; she knew Riker had a way with the ladies and was teasing him. I don't know how you can misread that if you pay attention to their relationship through the series, but whatev.