Boyfriends sex dream. Without me.

My boyfriend told me he had a dream where he was having sex with his old highschool friend (who is a girl) and I caught them in the dream. I feel really upset. We are each others firsts and now he has these images and feelings in his head of some other girl. I keep imaging his dream and how he now knows what its like to have sex with someone else. Yes its a dream but those can feel VERY real. He says he thinks the only reason he had such a dream is because we have gone awhile without having sex. I told him yeah, but you didn't dream of having sex with me sooo?