Dating someone doesn’t mean other attractive people disappear. As long as they dont interfere with a relationship you can acknowledge them. I don’t think you did anything wrong, your boyfriend seems a little self conscious, and I’ve been there before so I get it. But usually it’s better to just laugh it off or something then get defensive. Cause him getting upset isn’t going to make his girlfriend less attracted to fire fighters it’s just going to make her keep it a secret, and secrets usually aren’t great for relationships.