in life you'll meet principled people and not principled people. people who cheat are often not principled because they follow their urges.
you must also remember that we all have our own set criteria, you gotta define that criteria, and then ensure the criteria's of you and your partners' match before the relationship starts. if you don't then, as soon as your partner meets someone who matches their criteria more then they'll just be inclined to leave or cheat.