what should i do about my boyfriend and his sexual thoughts

My boyfriend once admitted to me that he once thought about his classmate in a sexual manner, that would have sex with her. He didn’t mean he would actively go and try to have sex with her but he just thought of sex with her. I am deeply hurt and have been feeling insecure. I love him very much but this has really hurt me. What should I do? I can’t seem to get over it.