There are people who think that an affair of the heart and mind is every bit as much cheating as is a physical affair. While I am tossed with this one.... I think it largely depends on the state of mind between you and the person you are having cybersex with and the people each of you are with.
I am a woman, and women tend to take a bit more to heart when it comes to having an affair of sorts involving the heart or mind. So I think this would involve talking to all parties involved before making this conclusion and seeing how each of you feel about it.
There are several different types of affairs and each person will view them differently. Not sure if this answered you really, but we had this discussion in a chathouse a long time ago and came to these conclusions.
Tam