Why is "fornication" used to describe the actions of "Babylon the Great" in Revelation 18? For because of the wine of the anger of her fornication all the nations have fallen [victim], and the kings of the earth committed fornication with her, and the traveling merchants of the earth became rich due to the power of her shameless luxury.”&mdash;Revelation 18:3 (NWT)


I was wondering if anyone would be able to explain, in what way(s), "Babylon the Great" engaged in "Fornication"?
In layman's terms, fornication is often used as an allegory for idolatry. In the same way people are unfaithful to spouses by fornicating in the usual sense of the word, idolatry is "cheating on" God. 

More at http://www.thewholetruthnow.com/2PORNEIA.htm and http://www.sabbathcovenant.com/doctrine/fornication_defined.htm

From the second link above:


  In Revelation, we again see a clear link between the act of
  fornication and idolatry:
  
  Revelation 2
  
  14   But I have a few things against thee, because thou hast there them that hold the doctrine of Balaam, who taught Balac to cast a
  stumblingblock before the children of Israel, to eat things sacrificed
  unto idols, and to commit fornication....
  20   Notwithstanding I have a few things against thee, because thou sufferest that woman Jezebel, which calleth herself a prophetess,
  to teach and to seduce my servants to commit fornication, and to eat
  things sacrificed unto idols.
  
  John is clearly speaking out against the practice of committing the
  sexual act of idolatry and then eating meat sacrificed to idols in
  that act of worship.
  
  If you do a word study on Fornication, you will see that it is
  condemned in context to Idolatry EVERY time.  100% of the references
  to fornication are in context to idolatry.


similarly, several times in Scripture, idolatry is described as "whoring after other gods" such as in 


  Judges 2:17 KJV
  
  And yet they would not hearken unto their judges, but they went a
  whoring after other gods, and bowed themselves unto them: they turned
  quickly out of the way which their fathers walked in, obeying the
  commandments of the LORD; but they did not so.