Well certainly one should not simply drop everything and run for the nearest exit at the hint of something a bit smelly in a relationship. That isn't what it is about. You should try and work out what was wrong and how it can be fixed. If you love someone you should want to exhaust all other avenues before resorting to leaving them.
Now I agree, that this behavior of hers logging on and creating a profile at a match making / dating site is totally unacceptable. I wouldn't tolerate that either.
But I certainly wouldn't go running for the door without first seeking some explanation as to why, and whether there is regret and if they are willing to change things.
Have you exhausted all options? Sat down and communicated like and adult your feelings and hurt at this?
And what was her response?
If so, and you still can't trust her and think there is no hope for a relationship to continue then move out by all means and begin to move on.
If you decide this is what is best then please repost letting us know of your choice and we can help you through the confusion and pain you will feel!