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1.
Sex that is so amazing that it seems fantastical, mythical, impossible, or unrealistic; sex that transcends conventional reality and transports the lovers into alternate dimensions; sex that most people do not believe exists, even if it is described to them by one who claims personal experience.
My date was so incredible, we totally had unicorn sex!
or

Yeah, I've been hooking up with different people, but now I really want that unicorn sex, you know?
by M. Svairini March 30, 2010
 
2.
the act of shoving ones horn into another ones anus. Then stabbing one in the asscheek after orgasming.
Honey! My pet unicorn can't wait much longer.

But the last time I had unicorn sex I went to the hospital.
by PoPoPoPoPoPoPo October 19, 2008