Determining the Unlawfulness of Child Marriages

Taking into account that most of International Law come from accepted culture and customs how can child marriages be accepted while pedophilia is totally not? This is the case in most child marriages happening still nowadays in many parts of the world young girls being forced to marry much older men. There''s no doubt that it all configure the crime of pedophilia in the end. If you think not you are as naive as an eight years old.