Quoting LumpySpacePrincess:" Lets just say for sake of different sides, one person is clicking with their partner in the same way, but another person in the same situation isn't."
I definitely think it's possible to find that closeness with someone else and still be clicking like that with your SO. But I also think that if that situation happens, you need to decide which person you want to "click" with and dissolve the other relationship. Your ultimate goal is to spend the rest of your life with your partner and share everything with each other. Not with the third wheel over there.
Now if it's a case of only clicking with the outside party and not with your SO at all, then SO needs to go. Obviously your relationship isn't as deeply rooted or connected as you thought it was and you're searching for something deep and meaningful somewhere else.