I know this sounds...weird, but I really don't think you should be put into a situation where you have to make excuses, promise to delete all "offending" material and then forever sneak around deleting history and panicking if she uses your PC forever more. I really think you need to be honest with her. :S
You are right, a very large proportion of the population watch pron occasionally and it is there for entertainment. From what I have heard from the majority of guys, they watch porn in happy healthy relationships as well as when they are single. It has no bearing on your relationship, or how much you love her, just like a woman picking up a vibrator, we do not "prefer" the vibrator, but it is just an aid to getting us to orgasm, same as porn for men. (being more visual)
I personally don't think orgasms or pleasure should be "tit for tat" I don't understand why, if a man or woman is feeling horny, but their partner is out of action for a week or so, due to illness, whatever...why that person should "go without" to prove they are willing to suffer? The only exception to this would be if the partner actually did want to have sex, or play and the other person turned them down in favour of pron, or playing alone. Then I could see a problem. This is the same for couples with mismatched sex drives. It is just as unfair to demand that the high sex drive person "just waits" until the low sex drive person is ready, as it is to force the low sex drive person into forcing orgasms to please the high sex drive person, but like "letstrythis" says, I also take issue to how you worded the last part of your post. There is nothing wrong at all with enjoying some porn when she is out of action, but to say "Well, its this or I go find another woman" is ....just....no!
Porn is not cheating, self pleasure is not cheating...but using it as an excuse to "avoid" cheating is low. Would you really consider going with another woman, if porn wasn't there? if so i think you might not have much self control!
Anyway, back to the point. I personally found this article, written about how porn is not cheating, really funny and helpful: http://www.girlonthenet.com/2012/03/05/on-whether-porn-is-cheating/
I hope you sort it out! Best of luck