To me, it is all very simple. Very very simple.

Why would a guy spend so much time watching porn? Why is he not having sex with real people, instead?

There must be some kind of a problem with him. Maybe he has social anxiety and cannot find suitable, fun partners in real life.

So he watches too much porn. BECAUSE HE WAS ANXIOUS TO BEGIN WITH.

So, reverse causation - it is not too much porn causes anxiety, but men with anxiety watch too much porn.

I agree with IT - a bit might be OK, but doing it too much means that there is an underlying problem with social interactions, prioritization and self-discipline (in IT's example, the guy has a problem with prioritization and self-discipline), maybe self-esteem, etc. etc.