"Unicorn" is being used to mean an unobtainably perfect boyfriend in discussion of the TV show Glee. "Andy" (from Cambridge, MA) started the trope by writing "I'm going to throw it out there: HE'S A UNICORN! Maybe I've been screwed over too many times in love to believe in good guys, but Blaine has qualities I don't see in gay guys."
This was picked up in LGBT Weekly where they asked: "Is Kurt’s romantic, sensitive boyfriend Blaine a figment of pure, unattainable fantasy?"
See also: the unicorn as the attractive single female swinger.