It sounds like your "Unicorns" are rather weak willed people. Weak willed people rely heavily upon the acceptance of others, so it should be an alert going off in your head when you meet yet another one.
For what it's worth, our unicorn wasn't into poly either when we first started talking. I didn't give her much consideration due to it, and merely just treated her like some friend. However over time the idea must have sprouted in her mind, as anything logical does to an intelligent person, and she become receptive.
Of course she isn't someone heavily reliant upon the acceptance of others, even though her mother is fine with it. Most of her friends and family aren't and she doesn't really care. If anyone did anything to wrong her or us because of it they would be punished, so that's the way I live. As George Bush says, Living in fear means the monogamists win.