Because they are not correlating porn with small brain sizes. They are not even correlating porn with a specific region of the brain being smaller. They are not correlating porn to anything, because they are using the word porn without a definition. As for motivation, you seriously need to ask that? The puritanically obsessed people in our society have been shunning anything sexual since before we were even a nation. See the entire gay marrage debate as an example of how people would feel it was their duty to control other peoples sexuality,.
Your analogy is still wrong. It is like a study showing people who steal are unattractive to the opposite sex, and then, sometimes, counting singing happy birthday at a child's birthday party, sometimes, including downloading a TV show off the internet, sometimes counting not watching the commercials when a you are watching TV, sometimes counting armed robbery of a Quickee Mart, and sometimes counting telling a joke at a party without finishing off telling everyone where you heard the joke.
You see, we are not talking about casual conversation. We are talking about something being passed off as a scientific study. Beyond the damage done by the specific study, it puts all science in a bad light. You ever wonder why people don't trust scientists? Because made up crap like this is consistently passed off as science. YOU are responsible people dismissing scientific studies. YOU and your kind are responsible for people not getting vaccinated. YOU are responsible for people not trusting climate scientist. Because YOU endorse and encourage bad science. Congratulations, that's what you get to look back on your life and be proud of.