I met my husband through role-playing. I had been in the group about a year when the GM (Game Master) suggested his brother as a member.
Since we were all in our early- to mid-20s, and since the group membership was pretty evenly split between guys and gals, the various campaigns soon took on an "adult" undertone. Much flirting happened. One memorable night we spent half a hour decided who was sleeping together in which tent for about 10 minutes of game-time.
However, something unexpected happened. None of the in-game couples matched up to any of the real-life couples. We were all experienced enough role-players to separated what our "characters" felt from what the "players" felt.
Over the years, this pattern repeated itself over and over as Hubby and I played with various groups.
Then, at a point when we didn't have any fellow role-players nearby, we discovered MU*s (Multi-User games - most famously MUDs). Although we both had characters in the same game, we only had one computer so we couldn't play at the same time. Our characters shared friends, but couldn't actually interact with each other.
One of the first things we learned about MU*s is that most of the players want to pair up their characters romantically. The next thing we learned is that quite a few of those pairings play at cyber-sex. So, one day we sat down and asked each other, "How far do we go?"
Because of our years of experience with role-playing we were both confident that the "play" wouldn't be real - in the sense of infidelity, but we knew it would be real - in the sense that anything you act out has an impact on your psyche. We quickly hit on a simple rule: "Don't do anything we weren't willing to let the other one read," with the sub-rule "Either of us could read the other's game logs anytime we wanted."
Periodically I run across articles asking if online sex-play is cheating. Basically, it can be, but it doesn't have to be. Do you try to hide what you're doing from your Real-Life partner? Then you're probably cheating. Does your Significant Other feel shut out and/or powerless? Then you're probably cheating.
But, if it's just an interactive fantasy that you're willing to share; if it's something you both understand; if there's no shame or secrecy, then I don't see how it can be cheating.