Fairly new GF asked me if I ever cheated on a GF. I said "no, but I was once hooked up with a girl that was engaged" [this was a long-term hook-up, once or twice a year (vacation house), then she got engaged and we hooked up once].
GF was like OMG, you don't care about marriage, how did you do that?!?! I said "SHE was the one cheating, it's not like I have to be the moral police, I was just the penis, she could have found another".
I believe the one engaged/married/exclusive has to be 100% responsible for maintaing faithfulness regardless of temptation. She says it's both people's fault if that one knows the other is supposed to be exclusive. That the one was preying on the other's weakness (she said the person getting married COULD have been vulernable/stressed, I asked how she knew, lulz?).
Lesson learned: Don't mention anything about past anything, even if it seems innocent, you can't convince someone analytical anything.