I did. With her best friend.
You see, let me tell you a story: It was a stormy night, my gf at the time (my ex now) and I were spending the weekend in her best friend’s fiance’s cabin up on the hills. It was just the 3 of us as her best friend’s fiance had been called back due to family matters. Always thought her friend was cool as we hung out a lot (even without my gf or her fiance) and liked a lot of the same things. Heck, I would easily say that we were much more alike than me and my gf was. Didn’t really have a lot of female friends at the time but I would say that she was one that could really hang with.
Anyway we were bored so the 3 of us decided to pass the time talking and laughing and drinking wine and playing cards. My gf played as the “bank”. Time passed. We’ve already had quite a bit to drink and things got a little heated. My gf had won almost every game and me and her best friend were down to almost our last chips and my gf, drunk as she was, loved to remind us how badly we were doing. I’m usually used to my gf being a little abrasive when drunk but this time she was downright obnoxious. Likely a tactic to make us do even worse at cards. I would often look to her best friend and nod apologetically and we would both share a smirk. It got to a point where my gf and I almost got into an argument as I was quite drunk myself and I wasn’t really all that able to take the sometimes rather mean jabs she was indiscriminately throwing our way. We were down to our last chips and were just ready to quit so we threw all our chips on the table and bet it all so we could get this game over with and do something else. We lost interest in the game long ago so her best friend and I were actually having our own conversation on the side talking about our shared interests in games and comics. Laughing and getting along so as to shake off my gf’s rude provocations. I’d say in that moment, we actually really clicked more than we usually did as we both had our shared annoyance at my gf. We went thru another bottle of wine.
My gf volunteered to open another bottle, groggily stood up and headed for the kitchen. We knew it would take her some time to get back as she could barely walk straight. At that moment, her friend and I looked at each other knowingly and just knew. We knew what we wanted to do. We didn’t even have to say it, but we did anyway. So I looked her in the eye, gestured for her to be quiet and finally said what we both were thinking: “Let’s switch cards out of the deck so we can finally win!”. So we did. We won that hand, said game over and much gloating commenced. And that is how I cheated on my gf with her best friend. That was many years ago. Til this day, she doesn’t know. And I’ve since lost contact with both of them. True story.