I would say that cheating goes beyond just the physical intimacy. I hate to admit that there are people that cheat on their partners emotionally and feel that it is their right to do so. Once you make any move towards breaking the trust someone has for you, then you have cheated.
Cheating is cheating, no matter how you define it and it displays a lack of respect for the one being cheated on. If a relationship has run its course, then end it as amicably as possible. It may not be easy if the feelings are not mutual, but it's the more honest way forward.