Its making me think of the times one of my friends would begin dating someone new and we (their social circle) would all get a bad impression of their new interest. Some of us would ask things like "geez why are you dating such a idiot/db/loser?" only to hear:
"When its just us, they are so different."
This can be a whole new ballgame when it comes to poly. Some people experience anxiety over the idea of the friends of someone they really like not approving of them. With poly, it isn't just the friends but also other partners - people they could easily worry might judge them poorly intentionally no matter how much they try to make a good impression just to get them out of the picture. Perhaps its similar to meeting the parents that think no one is good enough for their baby? There they are trying so hard to make a good impression that they become someone entirely different.