My boyfriend looks at porn and I don't want him to.
Why? Do you feel insecure about his behavior?
Sex is the only reason I started dating him in the first place.
It's normal for you to feel that way, but you can't control everything that he does. Try talking to him about how it makes you feel instead.
I can't handle it, we should break up and I should leave him.
Breaking up seems extreme but it is better than to try to control what your boyfriend does with his body in private. It is not your right to decide what others should do.