A great question posed at the Sex in Games blog: if you have a "liason" in a virtual world, does it count as infidelity in the real one?
If your partner would be upset by what you're doing, you've violated his or her trust. Whether it's "cheating" at that point is purely academic. The absence of a physical world doesn't make that violation of trust any less painful.
More on the subject from Apogeevr.
