We know with certainty that people forget traumatic events. Probably the best examples are of people's experiences of wars and car accidents. There are men who fought in wars who remember little of what happened, yet there is no question that they were there and that the war happened. Many people who survive serious car accidents do not remember the accident. People forget overwhelming traumatic events.
Many individuals develop trauma related reactions, even when they have no memory of the incident.
Many war veterans who have little memory of the war will suddenly duck when they hear a loud sound without understanding why. People will often have post-traumatic reactions even when they have no memory of the trauma itself. It's possible for someone to have no or few memories of sexual abuse, and to not have post-traumatic symptoms or to have only mild symptoms. Sometimes, post-traumatic symptoms don't develop until memories come back. This is why it is possible to feel relatively unaffected by sexual abuse only to have post-traumatic reactions emerge later when sexual abuse memories start emerging.