The most glaringly obvious problem with this study is of course its cross-sectional methodology. whether watching porn leads to brain changes or whether people born with certain brain types watch more porn,” Kühn told The Daily Telegraph (and yet that paper still ran the headline: "Watching pornography damages men's brains").
It's just as likely that men with less grey matter in their striatum are more attracted to porn, as opposed to porn causing that brain profile. A further problem with correlational studies is not just that the causal direction can run either way, but that an unknown or uncontrolled third factor (and others) could be causally involved.
The third scan looked at brain activity while the men relaxed in the scanner for five minutes (a so-called resting-state scan).
This is the same for anything that's addictive though and so it's important to keep checking that you are feeling okay with the amount you are masturbating.
If you feel comfortable with it and don't feel as though it's stopping you doing other things, then you should feel free to do it as often as you need.
The men also answered questions about how much porn they watch.
They averaged four hours per week, and none of them met the criteria for Internet sex addiction according to the "Internet Sex Screening Test". The researchers found that hours spent watching porn was negatively correlated with the amount of grey matter in a subcortical region near the front of the brain - the right striatum - that's known to be involved in the processing of reward (as well as lots of other things).