The Adult Content report option is mainly used when it comes to underage users, especially when they ask how-to sexual questions. "Posts that provide too many sexually graphic details by a member who is under 18 are removed for their protection. This is sexually related content that would not be presented in a high school Sexual Education course. For example, for those under 18, a Sexuality Question such as, "Why do guys get pleasure from nude pics?" is fine while "Best way to turn on a guy friend?" is not."
Reporting is just common sense - If you feel something shouldn't be up on a public forum for 13 year olds and upwards then you are entitled to report it then the site will adjudge the validity of the report and decide whether to remove it or not.