Porns threads on MN are rarely helpful to the OP.
They invariably become an opportunity for the porn haters to air their views and restate all the evils of porn vs the porn is no big deal everyone does it you're just uptight brigade to have a bunfight.
I think SGB is speaking the most sense on this thread for the OP. In fact, strangely, I usually agree with SGB on most sex issue threads, despite being married monogamous and happy.
OP - I experienced a similar thing. Thought our sex life was good, thought I was open minded, watched porn together in past...and then discovered he was looking at porn on line, and got really upset.
I too felt it was inconsistent or illogical of me to feel this way, but it somehow felt seedy for him to do this alone as if this type of sex is secretly what he's into. Not sure if that make sense to you??
We talked about it. In fact we rowed about is because he did look at it again a few times after I'd told him I was upset, and then when he realised it was a big potentially serious issue between us he stopped. I think before this he viewed it as something harmless.
I think you have to talk to him, try to convey how it makes you feel and why. Listen to what he says, let him explain why he looks at it and how he views it, I suspect he'll think it's not a big deal and will be surprised that it is to you.
Then decide together what will work for the both of you. Do you want him to promise to never look at porn again? Can he live with that? Will you feel happy if he tells you when he's looking at it so it's not secret? Will you be happy to agree that's it's only something you want to do together?
My DH and I have also talked a lot since about the dangers of porn, porn addiction, how destructive it can be to marriages, the effects on people in the industry and the pornifciation of society and the worry that has for our kids- and I think he's come to see it is potentially a damaging route to take.
Just get talking and you'll probably find it will bring you closer together.