Derogatory. An individual or couple seeking a unicorn or HBB. Different from a person who is merely attracted to bisexual poly people. Refers to someone who is seeking "that special third to complete" their family. They often believe a bisexual third partner will prevent jealous feelings on the part of either of the original members of the dyad because of the mistaken assumption that one will not get jealous if one gets to do all the same things as the other and no one ever experiences anything apart from the other half of the primary couple.
This type of couple expects their hypothetical future partner to be single or willing to give up any existing and future partners, to love & have sex with both members of the original dyad equally, and for each member of the existing dyad to reciprocate exactly an equal level of love and sexual attraction for the new person.
This type of couple will not consider any kind of relationship with a male, with someone who is only sexually attracted to one of the original dyad members, with someone who is already partnered, & usually promises to break up with the new person for the sake of "protecting" the existing dyad, leaving the unfortunate third partner feeling disposable. Sometimes the unicorn is expected to not develop any emotional attachment and is strictly there for a sexual relationship (equally distributed to both members of the dyad) and/or is prescripted as a secondary.
John is so insecure about sharing his wife with another person, that he thinks he won't ever feel jealous as long as his wife never does anything with anyone else without him also participating so he's looking for a hot bi babe that'll be willing to share them both equally and not make any waves. He's such a unicorn hunter!