I think it's because there are a lot of people out there who are sex deprived and all of the movies and tv shows out there are a big tease.
It might also add poor sex lives of many married americans.
Might I suggest a solution to my fellow Jews who are married or plan on getting married: Niddah. Meaning, don't have sex during her menstruation and the additional 7 days after not a drop of blood is found...it will be like one's wedding night...(as has been described to me...i personally don't know myself)
I don't know why anyone would want to have sex during a womens period, that's just gross.
Anyways, just ask the majority of teenagers around the world from 14 and up. Heck even when I started using the internet when I was 9 or 10 (1994 when I first got online), I used to get free porn emailed to me all the time. I remember loading up a picture and I was on a slow 33.6 K connection at the time, pictures slowly loading and my mother sees a horse and she's like "awww that's cute" then it loads up more and theres a naked lady on top of it and she's freaking out like "close that now!".
It wasn't hard to find porn at 13, and it wasn't difficult to get past the filters. Many of the Arabic countries and China have strict laws on pornography but yet if you go on paltalk, the majority of the perverts in the adult rooms are Arabic. It's really quite an impossible task to stop it. Just look at Israel, while pornography is illegal to sell/trade (in most of the country except Tel Aviv if I recall), it's still easy to find it, you'd pretty much have to ban webcams and heavily monitor every aspect of the web which is just...well nearly impossible.
I do realize the harmful effects of pornography though as it does cause one to think immoral thoughts about women and think certain things are acceptable that aren't, as the hardcore pornography gets worse, the more acceptable society thinks it is.
Oh not to mention, I believe masturbation is forbidden in Judaism to begin with, something about not wasting the seed and that it is worse than murdering a person if I recall? I mean what else would people be looking at porn for? I think it's fairly obvious