Once. And while I felt guilty as hell and hated myself for it, it turned out to be the best thing I ever did. Because it set off this whole avalanche of thoughts and emotions and really pushed me to reexamine the relationship I was in.
I've had long-term relationships before that I didn't end when I should have and I was worried it was becoming a pattern. I was feeling a little unfulfilled by my current (yes, still) relationship of then 4 years but I couldn't put my finger on where it was lacking. I would compare my girlfriend to my "ideal" woman or to some very desirable women I would meet, and her shortcomings were so insignificant. But still, I had a hard time really committing to the relationship.
Then I had a one-night stand with a co-worker after a booze-soaked Xmas party (such a cliche). Very beautiful and extremely sexy girl - someone all the men in the office drool over. The sex was awful, of course, being as drunk as we were, but that's besides the point. This girl actually wanted it to be the start of a relationship, I knew her quite well and I was able to think through the whole "if I were single" scenario.
And it all ended up in one giant epiphany. I had the perfect woman already, and not only that but we're aligned on all the other things that matter - plans for the future, etc. I had some major commitment issues of my own (divorce can do that to you) and I honestly think that had it not been for my being unfaithful I would have still been in that emotional limbo that was preventing us from moving forward in our relationship.
Anyway, so here we are, a little over a year later. We're engaged, expecting our first child, and I'm more in love than ever. I haven't told her what happened, nor will I. Nothing good can come of it. Despite all the good that came of it in the end, it was emotional hell, and half an hour of sex, even it had been good, could never be worth all the self-loathing and guilt I went through. I will definitely never even contemplate cheating again, I've learned my lesson, and I see no point in putting the woman I love through all the pain and anger and distrust it would cause, just so I can easy my own conscience.
15 years ago I would read a story like this and go, "nice post-rationalization, you cheating jackass" and I'm sure that's what most of the college kids on here will reply with. And that's fine, I'm not looking for any kind of affirmation from letsrun. But since you're reading this thread, I'm guessing the topic is of some interest to you, so take my story as another piece of evidence that the world is not so black and white.