I heard it was under the feudal system when the King had to approve marriages (where our need for a marriage license comes from).  After the ceremony, the couple was given a sign to hang on their honeymoon door - Fornication Under Consent of King or as we know it the acronym "F**K".  I don't want to digress into a George Carlin rant about this acronym being the most versitle word in the English langauge - non, verb, adverb, adjective, etc.  LOL