I just noticed this old thread. The writer obviously doesn't know there is an introduction to this song (or call it a first verse if you like) which has a different tune to the main part of the song. Many songs from this period had an intro. It goes...
You know its a sin to tell a lie
Still you keep saying "I love you"
It may be true, I wish I knew
But I'm giving fair warning to you
Be sure its true when you say ...etc.