The first one is based on brain scans, which is hard to refute.

Yes, but the participant selection was dubious.

Also, while brain scans are used, it's impossible to form a conclusion based on it.

For instance, do men with less grey matter watch more porn? Or does watching more porn cause men to have less grey matter?

A similar study was done on vegetarians. I don't recall the details, but it went somewhere along the lines of "vegetarians have more brain activity associated with empathy". Does that mean vegetarianism improves empathy? Or do empathetic people naturally gravitate towards vegetarianism?

Behavioral studies are so much harder to do compared to health studies. I don't envy the study coordinators!

But more data can always bring us closer to answers, so I'm glad that at least some informational gaps are being filled.

There's also a huge spectrum of consumption between porn addiction and adolescant curiosity. These studies seem to reference several consumption quantities which go beyond the scope of the original question.