Well... not exactly surfing for porn PER SE but surfing the internet increases brain activity. Porn sites are included, right?
Check out
http://www.worldlifeexpectancy.com/internet_brain.php to see the comparison of Brain Activity between READING A BOOK and SURFING THE INTERNET!
"UCLA scientists have found that for computer-savvy middle-aged and older adults, searching the Internet triggers key centers in the brain that control decision-making and complex reasoning. The findings demonstrate that Web search activity may help stimulate and possibly improve brain function."