I don't view porn as per se cheating if it's not hidden, kept secret, or taken too far. Absolutely if you're hiding or lying about it, it's a problem.
That said, there are aspects and forms of porn that I wouldn't automatically assume a significant other would immediately understand; forms of porn that would be potentially embarrassing just blurting out if she didn't already know.
Watching pre-recorded porn is probably the least intrusive form of "cheating" there's no interaction, no involvement.
Camming is much more dangerous to a relationship. There are emotions involved. I know the link Guy posted above makes it seem like women are more focused on emotional cheating and men on physical; but I can tell you that I would be sensitive to emotional cheating.
I made the "joke" once with a model, that I hoped someday to find a woman who would be cool enough to not just be okay with my friendships on MFC or CB, but would even join me to some degree in being a regular. As a man, and risking becoming emotionally attached to someone online in an interactive environment, I'd probably have to be very upfront at some point with a significant other, especially since it takes up so much of my afterhours online time.
In the highly unlikely even that I was blessed to be the significant other of someone who cams, I would probably have to constantly remind myself, that a)it's her job, b)if I love her, I should support her, c)communication is key d)when the cam turns off we are there for each other.
I think perhaps for men, a physical or body worker would be the hardest form of to deal with, combining both the physical and the emotional possibilities of cheating. From the stories I've heard of married or dating couples who work in the porn industry, they often have to set aside certain acts, even as simple as kissing, for their significant other, essentially as an anchor or signifier of fidelity.