Why are Catholics/Christians so offended by GaGa's song Judas?

I consider myself a Christian. I put God first, I am just a little more open minded than others. (With being gay, etc.) So when I heard the song Judas I was a little hesitant at first, but after someone explained it was really a metaphor, I didn't take offense to it at all. My question is, even though the message is not blasphemous, why are others taking this song so literal/harshly?