Its rather that once something like this comes out, the whole community will know. There's also a potential that children could access whatever pornography is being advertised. I don't think its contradictory - once its public knowledge, it becomes potentially or actually harmful.
Further, I do think that what people do has the potential to affect their mind - how they see things and behave. This is true of soldiers, or abbatoir workers, for example.
Apologies for making you angry, it wasn't intentional.
Its not so much whether sex work is or isn't immoral, or unethical, I'd consider that a separate discussion, but rather how that practice relates to children and their education and development. Something can be ok for adults to learn about or engage in but not for children.
As an example, its usually seen as not a good thing for children to learn about being a soldier (even if it happens in practice), despite it being a very good way of making soldiers, to teach them young. But the resulting harm to those children and society makes it generally outlawed, and certainly against public opinion. This is seperate and distinct from an argument about whether its good or bad, right or wrong for an adult to learn about being a soldier. The same applies to drug use - you need to be wiser and better educated than a child to engage with it, because of the risks and harms involved.
edit; to further clarify, with the soldier analogy, you might be ok with it being taught in a structured and carefully thought out way, but not for children to be watching war footage, if you see what I mean.