Many people are cheated on by the narcissist. You might think at the beginning of the relationship, in the romance and love bombing stage, that you will be immune to the narcissist cheating on you. He will not admit that he has cheated on other partners that came before you. You have no knowledge of his history before you met him, so you have no predisposed idea that he might cheat on you at all. So, you proceed with the relationship with the narcissist, as though you have met your twin flame, your soulmate, and your night in shining armor. He makes all these lifelong promises of romance, a perfect future, and just the two of you being together. You seem to have a relationship that, although not perfect, might at least seem palatable. Sometimes, the narcissist is everything you imagined in your fantasies of what a relationship should look like. Other times, he is your worst nightmare. Still, the relationship marches on.

Then, seemingly out of the blue, the narcissist cheats. You don’t realize it, but all the while throughout the whole relationship, the narcissist was cheating. He was doing little things here and there that might have given you clues that he was cheating. He covered many of them up. He couldn’t let on to you that he was cheating, or that he was grooming another mate, or that he was looking for your replacement all this while. After all, he views himself as a "good guy", and nothing you do or say will change his view of himself. 

The truth of the matter is that the narcissist cannot be monogamous. There are many reasons that he cannot be monogamous with you. Almost none of these reasons have to do with how smart, pretty, and successful you are. It has less to do with who you are and how well you are doing, and more to do with the way that the narcissist is wired and how he interacts with his environment.