I have talked with my gf about that. We have make a compromisse:
When we watch together a porn, we choose one, what we both like. The other we watch alone, but we talk about, what we like.
Its like watching a movie, i like digimon and she not. We dont have to share all things together.
So i delet always my browser history, i say to her: "i thing some porns u dont like, so why u will see them?". sometimes she gets curious and watch some still with me.
Every man watch porn, a normal thing, so the best is (i think), akt like is it a normal thing. As long the sex with u gf is good, its all okay.