These painful moments are not brought to you thanks to polyamory, but rather courtesy of a deeply sex-negative puritanical mainstream culture, which would have your three girlfriends conform, even if it means misery to them.
I think the lesson to take away is to be circumspect with the people you're out to. Discovering poly feels so amazing and wonderful, and when you're in love you want to tell the world! But some people have to be eased into the idea.
Your unicorns might have been more successful telling their exes/parents about you had they introduced you first, and had fun family times with them and you, their "special friends," together. Once the parents know you and like you, then you can start coming out to them in stages. But dropping it on them like a bomb before a relationship has even started--- the startled and frightened reactions are hardly a surprise.