Because he loves you!
I'm very glad he tells you how beautiful and wonderful you are - that's important. And, I think he means it too!
The cruel irony of time is that as we age, what we find attractive (visually) doesn't age with us. If you think about it honestly, are you more attracted to a 30-year-old with chiseled muscles or a 55-year-old man with wrinkles who's starting to sag?
The issue is that women often override their thoughts about what they find visually stimulating and focus just on their partners. Unfortunately, most men don't. They are more visually stimulated and the vast majority of them use pornography. And, as you know, pornography focuses on young, young women. This doesn't mean that he doesn't love you and it also doesn't mean that he no longer finds you attractive. My hunch is that he both loves and desires you.
My other hunch is that you feel most upset by the pornography because it reminds you (like you said above) of what you won't be again - not because it actually threatens the integrity of the relationship.
So, the question is, do you want him to abstain from all pornography while he is in a relationship with you? If it upsets you enough, this might have to be the ground rule that you set. However, I am wondering if that needs to happen?
I welcome your thoughts...