My partner and I have had the conversation about cheating and both agreed (as hellion did) that if either of us did that, there'd be no going back. However, I do know people who have had affairs and have been found out and have managed to stay together. I think if you both think you can and want to get past it, then it's probably possible.
I personally couldn't move away from the thought (and I'm sorry if I'm spelling thing you've already though of) that he thought little enough of you and/or your relationship, to to go off with someone else. And this is an affair - not even a drunken fling (not that I'd forgive that either).
There's also the fact that I wouldn't be able to stop throwing it into every argument we ever had after that, if we did stay together "HIM:I can't believe you did that. ME: Well I can;'t believe you had an affair". It's an ace card that'll win pretty much every agrument (unless you've had an affair to, which'll put you on an even) but the dynamic of the relationship'll never be the same again.