"Natural"? Well, pornography itself isn't "natural", it's very synthetic in how it's produced.
I do think it's natural for anyone to be attracted to other people no matter the status of their relationship.
I do think porn is okay, it that's what you're asking - to watch, participate, whatever.
My partner and I do watch porn. Sometimes we show each other what we like. Other times, it's a tool that's used for arousal when the other isn't present.
I do think that if porn interferes with intimacy - like Neo (I think?) said earlier, that's a problem.
As for, "give guys a break, everyone does it"...
Well, here's the thing. Is it reasonable to expect your partner not to watch porn? Sure, fine, have whatever standards you want. But you run the risk of severely limiting your dating options over something that's not that big a deal.
I'd encourage women who feel odd about porn to watch some moderate, soft core things themselves to see that it's not that bad. If you still don't like the idea of your guy watching porn, you have to accept that that's potentially a deal breaker in some relationships.
and when I say "you" I don't mean specifically you, just... in general.