Let's face it, there are degrees of immorality.
Twenty years ago, I would have labeled prostitution immoral without batting an eye. Now I've softened on that. But regardless of what those on this site think, society continues to see prostitution as immoral and it remains illegal. However, assuming the woman is not coerced, its a voluntary, informed act on the part of both parties. It can also be beneficial to both parties too. So, in my mind, mongering and prostitution are ranked fairly low on the immorality scale.
However, when you film someone without their knowledge or consent and post it on the Internet, you turn a private act into a public one. That risks doing great harm to the person who is unknowing being filmed. That includes both psychological (extreme embarrassment + its on the Internet forever) and physical harm (like if the girl's SO sees it, recognizes her, and has a violent reaction). And, as others have pointed out, its also criminal. So, its kind of a slam dunk that filming someone fucking without their consent and posting in a public forum (Pornhub) is more immoral than garden variety prostitution.
It's not that hard to understand.