An obligation to have sex due to pressure or circumstances. One may experience this when a friend is fornicating with their partner and you are alone with a person of the opposite sex your friend "set you up" with.
this is the expandedabbreviation of the word or slang term which is FUCK.
a long time in England, one would need to get 'consent' from the king, if a couple would ever want to have sex or have a child (this is not including nobility or royalty)
man:"Lets have a child"
Woman: "But first we must get 'fornication under the consent of the king.
use that next time you want to swear :D