To start with, smut magazines have been considered cheating, as well as strip clubs and phone sex. However, pornography is a very fictional relationship with another person. Usually there is no relationship to speak of, only the fantasy of one. Consequently, porn isn't very often considered infidelity.
The more interesting case, at least from our perspective, is cybersex. At what point does it become cheating? Does idle flirting on a chat channel or on instant messaging constitude cheating? Well, that's almost like flirting in the RealWorld, even if the person is more confident with themselves online. Most likely it's just harmless ego building. What about text-based cybersex with a stranger? While the porn industry is full of professionals, perhaps cybersex online with another John Doe or Jane Doe is pushing it. Much more seriously, many people have very intimate online relationships, although without the real physical component. Some try to pass this off as legitimate play, but it much more likely indicates your current "real" relationship is seriously lacking in depth.
In any case, the main rationalizing in most of these cases is that the relationship is mostly text, and therefore abstract. However, as the fidelity of the medium increases, it's unclear whether people will be prepared to accept the relationships as inoccuous. Certainly webcam sex already raise the level of the relationship to much more intimate levels (as the veil of pseudonymity lifts), however it is still action at a distance. When actual physical contact can be replicated over the wire (and there are already devices that attempt this), the mental "Chinese Wall" between play and cheating will further melt. And if VirtualReality ever becomes good enough to fool one's senses into believing it is real, the question of cheating will really come to a head.
Essentially, the point is that while the basic truth of the infidelty doesn't change (as that is an internal emotional issue), the ability for the participants to lie to each other about the legitimacy of their actions or the purpose erodes as the fidelity of the medium increases. In other words, in these caes, the low fidelity of the representation creates confusion in contrast to the higher fidelity in the above cases.
In Episode 24, of Star Trek: Voyager, entitled "Persistence of Vision", Captain Katherine Janeway reveals her personal holodeck entertainment, which unlike prior leading male character Star Trek fantasies, does not involve some sort of resolution of conflict, such as Picard's hardboiled film noirs, Data's Sherlock Holmes, and Julian Bashir's James Bond-esque fantasies. As Murray says, Janeway's fantasy is more like a novel than a shoot-em-up, where the captain obviously spends a lot of time in the world--not just getting to know the characters, but becoming part of the world herself. For her, the enjoyment is not the linear progression to some outcome, but the experience of being situated in the world itself.
However, these attachments become deeper. She becomes attracted to the fictional Lord Burleigh, a holodeck character, who catches her unawares at the beginning of the episode by kissing her. While this begins to raise ethical and moral issues qua RepresentationConfusion and infidelity fidelity (what does it mean to be attracted to a fiction?), the moral question becomes steeper later in the episode.
In the way that Star Trek episodes often do, a telepathic alien force tries to take over the ship. The alien presence does the typical ploy to induce catatonia by giving the crew their deepest fantasies hoping they will lose their willpower. What's (almost) novel about this version of that familiar Trek plotline is that Janeway here is caught between two lovers. Between her fictive yet immediate, Lord Burleigh, and her real yet lost fiancé, Mark, whom she may never see again. In the sense that Janeway is starting to feel guilty for giving up on Mark, and hence her reaching out for a new--yet safe--replacement, she is trapped by this dialogue between her and the image of Mark the aliens present to her (transcript as qtd. in HamletOnTheHolodeck?, p.17):
While more or less could be read into this, it being a cheesy Star Trek episode after all, what is perhaps most important to draw from this is that people's conception of infidelity is routed in their sense of betrayal and guilt, i.e. how the other would feel, not in some mechanicistic description that X or Y behaviour constitutes cheating. Sometimes there is a mismatch in how much one feels betrayed and how the other feels guilty, and but it's in that dynamic that infidelity is negotiated.