I personally believe that people are leaving out some very important things here.
The first and foremost on my mind, as it sounds like you're planning on sticking with it, is figuring out exactly what triggered this. People don't just up and cheat, they cheat for a reason. She may claim that there was nothing that you could have done or nothing that you did to precipitate the event, but if you had, do you think she'd tell you?
Therefore, I'd say that if you want your marriage to continue and for both of you to be happy, you should absolutely get some sort of counseling, because while right now we're looking at this from your view, your wife sounds like a good person, so think about what she must be feeling: here she is, a mother of two and the wife of a man she's been with for 20 years, and she's let her emotions and selfish needs drive her to doing something put all that in jeopardy while causing the man who's stuck with her for so long an immense amount of pain. Don't forget that she's probably feeling a lot of shame and hurt herself right now.
Of course as I mentioned though, what is most important is the fact that regardless of whether or not she'd ever do that again, whatever it is that made her want to do it in the first place is still under there. If you leave that unaddressed, sure, maybe she'll never do it again, but that doesn't mean the desire or the emotions that led her to do what she did won't still be there. Obviously there's some need she had that she believed she could fulfill via doing what she did, and while I don't know your wife, I get the feeling that the need is probably still there. As I said, she may never act on it again, but if you love her and want her to be happy helping her work through that should be a goal as well.
That's why I agree with marriage counseling. We can all sit here and try our very best to go through the situation, but A) working through something like this is a 2 person process, and we only have access to one piece of the puzzle, and B) there's nothing we can tell you here that a counselor trained in dealing with this stuff couldn't. Getting over the adultery itself is only the tip of the iceberg. There's a whole, whole lot more down there that needs to be dealt with, and we probably aren't the ones to do it. Plus I feel that talking things out in a safe and assisted situation would help yourself and your wife better communicate your feelings which, in the end, could end up bringing you closer together than even before this incident.
Either way, keep in mind that the worst thing you could do is just forget about the incident or brush it under the rug. That's a recipe for disaster. Whatever it is that precipitated this in your wife won't just disappear, and the wound left on you won't heal without both of you as active participants.
That's just my thoughts on the issue. I'm a 21 year old dude who's barely dated let alone even thought of marriage, so take from that what you will.