Yum-Yum is supposed to marry Ko-Ko.  
The librettist was walking up and down the library in his new house when an enormous Japanese executioner's sword he had hung on the wall suddenly fell to the floor.  The event gave him the idea for this opera.  Nanki-Poo is in love with Yum-Yum, the most desirable of the 'Three Little Maids'.  He was supposed to marry Katisha, but submits to banishment instead.  Yum-Yum is supposed to marry Ko-Ko.  But in the end, Katisha ends up with Ko-Ko and Nanki-Poo with Yum-Yum.  FTP name this opera by Gilbert and Sullivan.
The librettist was walking up and down the library in his new house when an enormous Japanese executioner's sword he had hung on the wall suddenly fell to the floor.  
The Mikado