Why don’t you like your partner watching porn?

This isn’t a debate about who does and doesn’t agree with their partner watching porn, I’m just wondering why specifically people don’t like it? Just for insight/my own interest really. For me personally, it makes me feel bad about myself, I never have and never will look like the girls he’s looking at and I feel like he is choosing looking at someone else over actually being with me, which just makes me feel horrible in myself.