Confused on what to think

Hi all, my boyfriend and I have been together for nearly 3 years and he constantly reacts to naked pictures of girls I either went to school with or ex partners of his on social media such as Instagram and it makes me feel so insecure about the way i look as I've noticed these girls are a lot smaller than me. Each time he does it i will speak to him about it and then he promises never to do it again, I then brush it all off but give it a month and its happened again.. I've never been the type to be upset over my partner having female friends I just feel that going out of his way to like naked pitcutres of people me and him both know is too far.. Am I being stupid, is this normal??