Will it ever happen to me?

Hi. I have been living with this fantasy for years but never lived it for real. When I have sex with my GF, it is the thought of another man pounding her that sends me over the top. When I masturbate its the thought of her orgasming under another man that makes me cum. I have had this fantasy for the last 10 - 15 years, all the way back when I was dating other girls. My greatest wish is that she will cheat on me and I would find out. But I could never ask her to relax with another man, she would hate the idea and it would make great damage to our relationship.
But I'm a man that know that I only live once, and I would live to try this setting, if only once in my life.. Tell me what to do.
I have pondered the idea of arranging it with a callgirl, that could play the role of being my GF. Take her out for dinner and go to a bar and find a man. To run the whole line with a nice girl that can play the role..
But off course, its a crazy idea.