You don't need to be a cheater to do this. You could figure out you're settling by going to therapy and learning more about yourself in another way.

As for settling - this implies that people are static and relationships are static and there is such a thing as the perfect person.

In reality "settling" just describes a stagnant, less than ideally compatible relationship.

There are many ways to get out of settling. But they don't have to require cheating. You could get therapy. You could pursue your dreams. You could both work on your relationship. You could become polyamorous. You could break up.

Breaking a commitment you make to another person, doesn't just hurt the other person, it hurts you. Because you didn't hold true to the moral choice that YOU made and you believed in.