Is it bad that other girls get in my head?

Me (21f) and my boyfriend (22m) have been dating for a year. Recently it’s become really hard for me to trust him. I’m not exactly sure why. But I get worried he gets sexual thoughts about other people when we are out. In the beginning of our relationship we were scrolling through social media, and out of habit, he clicked on a girls pic to get a closer look. She was obviously pretty. It got in my head cause he obviously thought that and that’s why he clicked on it. I’ve seen him do double takes on girls when we are out. He said he didn’t even know he did. And he says he is allowed to look at people and he isn’t attracted to them. Idk because it’s honestly just like stressful for me to see him looking at other pretty girls. It’s gotten to the point where I can’t even watch certain shows cause the girl is so pretty or “hot.” The idea of him being attracted to other people wrecks me. It’s put me in a bad mood and has ruined going out. I don’t even want to go shopping at my stores with him cause there’s so many pretty girls all around. Maybe it’s an insecurity thing. But even on the days I feel super pretty and good about myself it still gets me. How can I get better about not letting it affect me? Is this normal? It’s so draining.