you have to explain to her what porn means to a guy...
what do guys think when watching porn?
"damn... that's fucking hot..." schluq schluq schluq... SPLOOOGE!....
"i'm gonna take a little nap..."... "i'm hungry... gonna grab a bite to eat"..
basically, we think nothing of it...
now, if you go into a woman's head... it's totally different... they don't understand that we think nothing of it... they ASSUME this is what we think:
"wow... that girl is hot... i want her to be my wife... i want to ditch my GF/wife and be with this woman for the rest of my life... now that i see this naked whore getting rammed, i want to leave my GF/wife because i don't like her anymore and she doesn't turn me on at all... why did i waste my life with her?" ... etc...
seriously, this is what most women assume goes through our head when we watch porn.
sit your GF down, talk to her about how men feel about porn... that it's just like watching a movie that gives you a boner... and not a life changing epiphany where you'll stop finding your GF/wife sexy or she doesn't turn you on or whatever...
don't try to bring in metaphores... like "imagine eating egg whites all day everyday... you sometimes wanna eat a burger every now and then... this is like eating that burger but without cheating"... if you say that, you'll open up a can of worms... mean understand metaphores, most women don't... they'll just say "YOU COMPARE ME TO EGG WHITES!!! HOW COULD YOU, YOU INSENSITIVE PRICK!!!"... so, yeah, no metaphores
if she still doesn't understand... i would say to just act stupid and say you will stop doing things for her that you don't understand but still do because you know she enjoys it... like going shopping with her, buying her flowers and chocolates, celebrating valentines day... etc.
JMHO