I know this is an old thread but I followed a link
here from someone who found it useful and thought I would add my "two cents" - just to add to the collection of experiences for any future readers.
Although I have always identified as poly the "rules" (boundaries/agreements/whatever) of my relationship with my husband precluded sexual relationships with other men...but the line of what constituted a "sexual relationship" had flexed and stretched over the years (decades) and never been seriously strained...until Dude (MrS's new best friend) entered the picture.
For three months I was, essentially, a "cheater" for the first time in my life. Although we did not have sex "technically" - boundaries were certainly crossed that required some serious self-delusion to justify - I was so good at this that I managed to convince Dude, as well, that my self-delusions were true - although he always had his doubts. (You can read the whole sordid tale in my "Journey" blog on this site.) I can't even fall back on some "emotions got the better of me" argument - for me, it was all about the physical (on my part, at that time).
When MrS found out my deception ... the shit did hit the fan. I had ALREADY decided that I needed to cut it out with Dude and come clean with my husband. I was deciding how to do this when it came out anyway (due to my utter inability to ACTUALLY deceive someone I care about when asked a direct question.).
After the dust had settled...both boys forgave me for my temporary insanity and things have been ... well, actually pretty awesome since then. (Forgiving myself, on the other hand, is a much longer endeavor.)
I like to think that a one-time period of bad judgement doesn't define me as a person - but learning that I had the CAPACITY to behave in such a way was certainly an eye-opening experience. I like to think that I have learned from this experience and would NEVER behave in such a way again...but now I am leery of such absolutes. (For others, as well as myself.)
We are human. We make mistakes. We, hopefully, learn from them. People can change (for better or for worse). Change can be triggered by events but, ultimately, it is an internal process.