Whither online relationships? This article discusses some of the issues, such as inside a virtual world like Second Life.
If someone already has a real life relationship, I don’t see how this isn’t cheating. If someone feels s/he has to hide the relationship from the significant other, that should be a friggin’ huge red flag.
Although I have no personal experience, some people have approached me in WoW (my main is female). People will tell me from time to time that my character is sexy and/or try to flirt. One guy even tried to “get it on” in an auction house with scores of other people in the room. I didn’t respond. (Being somewhat kid-friendly, WoW does not have any mechanisms for nudity or sexual actions. But you can’t stop words and emotes.)
Only one guy has ever bothered to ask whether I were female in real life. He actually seemed very nice, and I felt a little bad for dashing his hopes. Yes, technically, it’s an assumption that he was male in real life.