Most people have those thoughts. The true sickness lies in fear and obsession of the thoughts. It's hard to explain really, I'm not a doctor or a psychologist, nor am I good with words.
Basically the best way to handle them is to acquire a dark sense of humor, so then when they do cross your mind you don't give them a significant sense of importance as you would if you fear and/or obsess over them
Me: *sees really old, overweight guy*
Brain: "I'm gonna imagine what it's like to have sex with him."
Me: "Nooooo, brain, pls stop."
Brain: "I'm gonna do it."
Me: "NOOOOOOO!!"
Brain: "I did it; it is done."
Me: "WHY, BRAIN, WHY?!?"
Thank you. I needed to hear that. Its tough not feeling like a monster do to those thoughts. Little solace in the whole "yeah, but I'd never act on them".
God I'm terrible
https://youtu.be/U6nuxmV6Zko
Brain: "I'm gonna imagine what it's like to have sex with him."
Me: "Nooooo, brain, pls stop."
Brain: "I'm gonna do it."
Me: "NOOOOOOO!!"
Brain: "I did it; it is done."
Me: "WHY, BRAIN, WHY?!?"