joanp: Fantasizing is not a sin. To NOT fantasize can be sinful.  A world without fantasy is a world without poetry, music, dreams, literature, art, and so many other things. Trust folks to fantasize. And to fantasize fully.   

One of the most damaging and unhealthy things I remember about Catholic school was the repeated stern admonition: "Boys, put a brake on those fantasies." The "take a cold shower" doctrine often was then cited.   

The Big Error: 

The beauty of fantasy and the sickness of lust were homogenized as equal evils on the slippery slope to Hell, and maybe, just maybe, that approach was sinful and unhealthy in and of itself.  To repress fantasy is to live life with the brakes on.