@nephalem,
In a sense it's indeed virtual infidelity, but more enlightenedly it's just a healthy sexual appetite. We shouldn't expect to be the beginning and end desire for our intimate partners. No one has everything in one body afterall. So seeing the variety and possibility is not just natural and normal, but healthy. Think of it another way, if everyone was naked all the time you'd be looking and fantasizing too but it wouldn't then be unusual to look and admire others. By virtue of our all being clothed though what we enjoy seeing is hidden which only makes it more alluring and desireable. That we call porn "porn" in the first place denotes a disapproval of it. Does mean afterall 'language of whores.'
Instead of seeing porn as a threat to our relationships we should demystify it and accept it more. Growing up most children experiemnt with smoking, drinking, drugs, and sex for no other reason than they're all forbidden. Forbid something and you only increase the desire for it. Make it available and acceptable though and it become unremarkable, less desired, and consequently, less of a problem.
If you forbid your partners something all you do is force them to consider leaving you to pursue it, or pursue it secretly. Allow them anything and everything (within reason hehe) and they have options. And often times having only the option can help us refrain from pursueing things. Like that last cigarette in the pack. Knowing it's there you may be able to go much longer without smoking it than if you know you're out and have to get more.