It isn't the easiest decision to make, and can sometimes be very complicated, but don't let those complications stand in the way of your happiness.  The decision really boils down to this.  If you can forgive them, and still truly trust them, then you should give it another chance.  If the trust is gone, and you find yourself wondering all the time if it will happen again, it's probably time to go.

"Forgive and Forget."  It is such an overused phrase, but it is entirely appropriate for this situation.  While I find it nearly impossible to forget, forgiveness is often possible (and essential) if the relationship is going to continue.  I don't mean saying "I forgive you."  Forgiving someone means that the incident becomes part of the past, as if it never happened.  It can't be brought back up later if it's been forgiven.  Now, you should most definitely talk to your partner about, preferably as soon as possible after you find out, or else it will just burn inside you, and when you finally do get the courage up to say something, it most likely will be in a adversarial tone, and a fight will break out.

Let your feelings be known.  Listen to what they have to say, too.  But, once it has been discussed, and everyone's feelings and thoughts have been expressed, then you must make your decision on what to do next.  If you find in your heart to forgive, then it is a dead issue.  It cannot be thrown up in someone's face in the future as artillery in an argument.

On the other hand, if you are not the forgiving type, or the pain and betrayal is just too deep, you may need to consider ending the relationship, and the sooner the better.  If you do not, the pain and bitterness will gnaw at you night and day, and will make the inevitable breakup even more excruciating.  You may think the relatinship is too complicated for such a drastic measure, but trust me, the longer you wait, the more complicated it will become.  If you are in a situation where you feel you may be in danger, enlist the help of local authorities, and as many friends and family members as necessary, to help you move out.  But the bottom line is, if you cannot let it go, or put it completely behind you, you must move on.

Basically, you have to put your feelings first, and do what is best for you.  Look deep inside yourself, and decide whether or not you can forgive the infidelity.  If you truly can, your relationship should last through many trials and tribulations.  If you cannot, you need to move on, and find your happiness elsewhere.