Porn can never be eradicated completely but banning it will deter people from watching porn. The argument for not banning porn seems similar to the argument that leftists bring out when people advocate stronger border protections and increased border security. The cost of enforcing a ban on anything immoral is often expensive and never full proof but the fact that it deters helps in the long run. Saudi Arabia might have a lot of people watching porn, but it probably would be higher if they didn't ban porn.

Also you can't really stop at banning porn, you also need to consider banning sexualized depictions in media and soft core porn. Japan for example banned child porn but they still allow the sexualization of adolescents in their art and media. There is some articles that talk about Idol companies selling dvds of child idols in swim suits of girls that are grossly underaged. Also, fetish porn can hide itself in plan sight like Nickelodeon show creator, Dan Schneider. He shows depictions of children displaying feet way too often not be considered a foot fetishist. There was a podcast of an actor not realizing how weird it was until someone pointed it out.