If you're once a cheater are you always a cheater?

No: But it is true that once you have done something like that you can be more easily tempted to do it again - unless - you commit to learning why you did it in the first place. Make the right changes and set up road markers and barriers around you to prevent it from happening again. If you are the one who is being cheated on, you can encourage the cheater try doing the above.

Yes: If you have cheated once, it means that there is a need or desire in you which was unfulfilled. So there is a good chance that the person will cheat again unless this unfulfilled need or desire is expressed to the partner. The excitement and the thrill of cheating can be intoxicating as well