I have never cheated on my SO.
Same here.
We're separated atm, but still no cheating. She knows I occasionally have sex with men, so that's not exactly cheating in our relationship... as long as I'm up front and honest about it, and I always am.
While we are separated, I might be convinced to do something with a pretty young girl from another office... but probably not. The more I consider it, the worse I feel about it.