I guess I can say I was cheated on by the woman who is now my wife, before we were married. For various reasons, we were not engaged but had been monogamous for over 2 years and had lived together for one year.
Requirements of her work and my training required us to live apart for a year, but she was coming to see me to spend every other weekend together, or so. We had made arrangements to work together the coming summer and knew that we would be living in close proximity to each other in the coming years.
That spring, I could sense from her attitude that something had changed so I confronted her and asked her if she was fucking another guy. She confessed that she had started a relationship with a guy she worked with and was having sex with him. I gave her the "him or me" ultimatum and she told me she wanted to end our relationship. We were both locked into spending the summer together, which was stressful to say the least.
After that summer I didn't see her again for over 3 years then we had a chance meeting and wound up getting back together and eventually marrying.
At the time I found the whole thing sickening and to say I was angry was an understatement, but I was prepared to forgive her if she stopped seeing him. She wanted none of it, however, and felt that she hadn't cheated since we were not engaged.
Now I really don't have any residual anger or resentment about it although I still believe what she did and how she handled it was wrong. I don't think I could ever do the open marriage thing, let alone voluntary cuckolding, but I must admit that I find the idea of my wife (then girlfriend) banging this other guy and then getting in the car to drive several hours to come down and screw me to be somehow titillating.
I guess it is the excitement that some guys get from seeing their girlfriend behaving like a whore?