I think forgiveness is important to you, not the person you are forgiving. I can always forgive anything, forgetting and allowing the person back into your life is something different altogether. Just remember that holding all of the pain and anger inside of you only hurts you, not them.
Personally, I don't think I could forgive it. I might could "get over it" in time, meaning I could get to a point where it didn't hurt me to think about it anymore, but I don't think I could ever trust her again. It would most likely be the end of our relationship.
Some research say it's more about the sexual jealousy than it is with betraying trust. Whoever finds the wisdom and strength to separate these two will have no problem forging a carnal betrayal, that is if betrayal is of carnal sort.
I would be able to forgive pretty quickly, because I try not to hold grudges. However, it would not be something that I would forget about very easily. I don't think that I would stay with someone that is cheating or has cheated. I couldn't trust that person.
My wife and I have an open marriage so this is not something that we will ever have to worry about. Even if we didn't have an open marriage I wouldn't throw out our whole relationship just because she had sex with someone else.
I think that it depends a little bit on the situation. In most cases, I could not move on in that relationship. I won't say that I never could, though. There is a chance that if there were circumstances that created something like a perfect storm, and she was very repentant, honest, and wanted to work hard to regain trust, I can say that I might be able to move past it.
I don't know that I would forget but I could possibly forgive depending on the situation. It's really hard to know what you would do until you are in that situation. Initially I don't think anyone forgives, it would take some time to work it out.