The definition of a unicorn in this instance is a single women who is prepared to play and/or live with a couple.
The term unicorn is attached, as they are rare or mythical creatures that don't exist. I was one once, until I met my partner and soul mate. Considering we are rare and mythical, we are in great demand!
A unicorn can live out every fantasy she wishes, as the opportunity and the offers are always there. A true unicorn knows herself, is comfortable in her own skin and most importantly, will not consent to any situation she considers to be detrimental to one of the parties involved. She posesses great natural empathy.
Yes, a true unicorn is a rare and precious find.