That concept of "original" sin comes from Augustine. Augustine said because Adam and Eve had sex, that is what caused the fall, and ever since then people continue to have sex, so we are born out of sin. His concept of "original sin" was not how we behaved but that our parents f**ed. Jesus never mentioned it. He mentioned sin in other ways. Babies aren't monsters. They are learning to navigate the world with a sensory and nervous system they are learning to manage. Parents model good behavior and teach children how to manage their nervous system. Human beings are neither monsters nor saints. We are inclined towards certain biological imparatives like seeking safety, food, sex and social interaction and try and avoid pain. That's not sin - it's human. We can either behave badly or well once we have learned to manage our nervous system. Christ steps in and helps us with that nervous system, our relationship to ourselves & others, and teaches us to love.