I have never cheated on a g/f, and to my knowledge the closest to being cheated on I’ve come is that an X g/f cuddled with another guy, and made out with another guy in a drunken stupor after we had a huge fight.
I have however helped a girl cheat on her boyfriend. It started out with us just being friends, we had a ton of mutual friends and we all hung out all the time. Then she and I started hanging out alone. We tried pretty hard to keep it on the up and up, but her boyfriend lived a few hours away and I was definitely falling for her. Eventually things got somewhat out of hand. We never had sex, but things did progress a ways. There were huge conflicts with one another about her leaving her boyfriend, or us just finally not doing it anymore, etc, etc. She was a very “passive” girl and was afraid to make changes on her own, so she would never break up with her boyfriend. Even though it was apparent to me and her friends that she was unhappy with him for several reasons (and this is even before I entered the picture).
Finally one night at a party another girl and I hit it off flirting. The girl I had been quasi-seeing was somewhat drunk, and she came over and interjected herself into the conversation. She didn’t say anything mean but she put out a very possessive aura with regards to me, and it turned the girl I was flirting with in the other direction very quickly. It basically occurred to me at that point that this needed to end. Not only was she not leaving her boyfriend for me because of her own passive/scared personality, but she was going to hinder me from moving on to someone else who wasn’t seeing someone. So I killed it the next day. I was kind of mean about it (in fact she was so upset she has barely spoken to me since, despite still having the same friends and what not… and this was several years ago now)... But it needed to be done.
But really I suppose this was less in the context of an affair, and more in the context of relationship issues. So I dunno how well it fits your description.
It was very exciting though, at times at least. I mean our friends saw what was happening, and with the exception of my room mates they weren’t very keen on the idea of us cheating on her b/f. So there was the aspect of keeping it secret from them. There was this aspect of would we make some kind of a move. Like our first kiss almost came about a dozen times. She would pull away one time, I would pull away another time, people would be around. One time we were hanging out at my place and I was joking around like covering her eyes for some reason, and I was finally like, “well no better chance than now”... and I just kissed her, and it was one of the most passionate kisses of my life. On top of all that there was still all the regular excitement and joy of a new relationship.
There was a fair amount of shame in it too though. I had always told myself I would never help a girl cheat, or cheat myself. It was one boundary I never wanted to cross. I wanted to be a better person than that and always believed I was. But here I was in a situation with a girl I was falling hard for, and I simply couldn’t stop myself. And every time I found the will power to avoid her or something, she would go out of her way to come to me because she couldn’t stop herself. It was like being helpless almost. There was one time when we were making out and being semi-intimate, that she just suddenly broke down into tears, realizing how wrong what we were doing was with regards to her b/f and our situation. It was a bad situation that I wish could’ve come around under totally different circumstances.