Why does my boyfriend watches porn while our sex life is healthy ?

Me and my boyfriend are in a healthy and happy relationship with no issues in our sex life. I watch porn maybe once a month because I prefer to read smut- erotic books.

We are long distance and we see each other every 2 weeks. He says watches porn 2-3 times a week. I know that it’s in our nature to get turned on by watching others having sex.

Just the thought of him getting hard with another woman’s body (looking at her boobs, ass) makes me want to throw up.I talked to my therapist about it and she said it’s normal but I just can’t accept it. Thinking about him jerking off to other women getting laid makes me sick.

Yes I read erotic books but there are no images of real people. Maybe I am wrong.

I am just trying to understand I am really open to hear your thought.