Not all women think the same. As a woman, I'd say tell her. Some women are stupid or emotionally (or financially) dependent enough to forgive. If you have young children, that might help you too.
I personally wouldn't forgive and much less forget, for all the reasons already listed on the previous replies. I'm just saying that I see it happening a lot; some women just don't have the courage to walk away.
Regardless, bottom line is, SHE HAS THE RIGHT TO KNOW. If you made a commitment to marry her, it means plain and simple that other women are out of the picture. Be a man and honor the commitment you made with her. You can't pick and chose when to honor it or not. You either are faithful or you are not, there's no grey area.
If you don't follow the commitment you've made, she has the right to know. And then decide what she feels about it and how to deal with it.