If I remember it right, one of the TO DO THINGS on the couple's LIST is to write a song and sing it together. During the Prawn Episode, Yong told Hyun that he has written a song for her. The WGM PD's was too quick to pick-up the situation and created a mission for the couple to write a song, sing it together and upload it on Youtube.
The story of the Banmal Song did not ended on Youtube as Yong wrote his own version of the song expressing his true feelings for his lovely wife. The title of the song was then changed to "Song For First Time Lovers" which was officially released under Yong's record label. The said song also topped the Korean charts (well if you're singing from the heart and you're singing for the one you love, I bet you're song will not only top the chart, you will place first in her heart).
What made the song extra special was that the other version that we've heard on the last episode of the GOGUMA couple. Hyun totally changed the lyrics of the song expressing how she feels towards Yong and answering the lyrics of Yong's version.
I know I'm being delusional but if you see it the way I see it, their last WGM episode was actually the start of of a new chapter of them being REAL. See how Hyun bravely sang SARANGHE to Yong and see the way she turned to him singing those words. She just answered Yong's SARANGHE. :)
Credits goes to all the owners and subbers of the videos that has been used to combine the 2 versions. Special credit to Youtube user orihararan.