Yup Natja, I believe that's the one.
It sounds great to stay loyal to each other as "the original couple" (perhaps thus not departing too far from monogamous ideals), but if you get the opportunity to bring a "third" (i.e. an additional woman) into your lives and/or home, be conscientious of her wants/needs/situation as an individual too. That's all. She's only a "unicorn" if you ask of her more than she can really give.
No flak intended, just passing on some of the info I have and agreeing with Ali that there is a certain stigma attached to a certain stereotype, yes, even though we're all polyamorous and one would think surely all on the same team. A lot of this caution is due to lessons others have learned in the "school of hard knocks." They don't want others to have to learn the same things the hard way.
Love means never having to say, "Put down that meat cleaver!"