Long time ago in England, a couple could not have a sex without a written permission from the king (Except if she/he was a member of the royal family). So if someone wanted to have babies, they will go to the king to ask for a written permission. The king then gave a kind of pass card that must be hung on the bedroom door when they had a sex. The card read “F.U.C.K.” (Fornification Under Consent of the King).
The more you know.