The term 'cheating' and its associated beliefs and emotions bring us straight to a morality conduct of what is deemed right and what is deemed wrong. But morality was born out of a social code of conduct which paradoxically was humankind's way of introducing bondage within societies. Nature is an intelligence of its own and if there had not been an inhibition of the natural flow of passion within every single individual ever since birth, the issue of whether it is ego-justified to have a sexual interaction with another person without your current significant other ever knowing anything would not have even cropped up. The bondage we have created and which we believe brings us loving support and ideals on which to create a stable society goes against a basic law of nature which is ... nature is such an infinite piece of intelligence that when it is allowed to thrive without bondage, it knows its exact limits. And within such a definition, there's no religion, no societal rules and no ego to provide conditional judgement; conditional judgement that leads to fear and guilt and a continual pattern of being in the 'wrong.
So your partner's having cybersex?
Question number one: Is your relationship thriving on unconditional love or is it based on a series of conditions?
Question number two: What is it about the flow of passion between you and your two unique lives that have led to your significant other desiring a higher level of passion through a sexual interaction with another person?
Love is about detachment and complete freedom of the other person in your life and it starts by freeing yourself of the notion that you are in a relationship that leads to the other person to being 'owned' by you.
Vi'
EDIT: Please have a look at this youtube video. It speaks about the nature of fantasies and appreciation and I think it's awesome!
http://www.youtube.com/watch?v=5XcmUVcfbNk