I've never been married to or had a girlfriend that I didn't give complete sexual freedom to. Not that they needed my permission, but just to be clear, I always made sure to tell them they could fuck anybody at any time.
All I've ever asked for in return is to share it with me. I'd prefer to watch, but if they don't want to do it in front of me, they needed to tell me everything, every time. I had no problem with them going on a date or going out with the girls, but I wanted to know what time they thought they may be home, so let me know if you were going to a guy's house for the night.
Most of them tried to hide it at first, but eventually would flaunt it, not caring if I knew or not.
And, yeah, knowing she is cheating is very exciting, even when some hurt feelings exist. Your mind recalls some of the best times of having sex with your wife, and are ****** to think about another man making her feel that same passion and pleasure. Perhaps more than you knew she was capable of.
You have to fantasize about all the different positions, how long/how many times, has he fucked her in the ass, has she sucked him off and swallowed his cum, does she take cum in her pussy, and a thousand other thoughts. And the next thing you know, your cock is very stiff, and you are jacking off furiously thinking of all these things just racing through your mind, and your cum just explodes when you shoot.
Then you decide that it felt good and want to replicate that feeling. And the only way to do that is start fantasizing about her getting strange cock, and the cycle above starts all over again.