@AnythingOld:
I don't know, I really think it depends on the relationship, and the relationship's ground rules. Personally, I did it once, and I had an overwhelming feeling of guilt, as he pointed out he new that I did. Mind you, I wasn't embarrassed that I watched porn; but I was more ashamed of my ex wrongfully interpreting that as saying he wasn't enough. But… That wasn't the case at all.
I had a similar experience in a previous relationships. One guy said that he didn't think porn should be used in a healthy relationship, so I threw all mine out only to find he had an extensive collection hidden away on his PC.
In another relationship I admitted to watching porn and was scolded for it, as the ex at the time believed it was wrong for me to even look at someone else, and made me feel guilty, so i ended up deleting 2TB of porn to keep him happy, but as it turned out he too watched porn secretly behind my back.
Turns out both those partners had control issues and were quite abusive to me anyway.
So this time around things are very different in my current (and hopefully last) relationship. We have complete honest and openness about such things, and it's a much healthier and happier environment for us both. There's no secrets or hiding, and no shame or embarrassment and no feelings of guilt, and we both have our private time if we want it.
But I agree, it's definitely depends on the relationship, and the relationship's ground rules…as long as the rules are not being secretly broken and there is actually honest and open communication about your want's, needs and desires.
Yes it can be hard at first to hear that the person you love finds someone else sexually attractive, but it can also be quite liberating, because if we are honest with ourselves, we all do it, it's just human nature.