Are unicorns real? (read below)

A Unicorn: a submissive HBB (hot bi Babe) who is willing to join a pre-existing D/s couple, with the understanding that she is secondary to the dyad.

I've seen alot of controversy about unicorns, whether they exist, whether it's okay to want one, if it's even real poly. The most common argument against them I've seen is that it's unrealistic to find a HBB to join your dyad, and that its unfair to expect the ”unicorn” to not fall in love with either person in the dyad, to be okay with being a secondary part of the relationship, and to walk away, no hard feelings, if it doesn't work out.

Personally, if me and my dom found a submissive bisexual female (there's lots of them), who we both liked, and we negotiated play with her, I would be more then happy. I think it would make since that she not be a primary partner in the beginning. I also don't see wants wrong or unique about wanting this? What's your opinion?