I'm glad someone made this thread, because I frankly find this topic fascinating, and feel kind of creepy bringing it up myself.
Until I was 16, I couldn't stand watching porn. It's not that it didn't have an effect on me, it's that it DID... and for some reason, I found that uncomfortable. It wasn't my upbringing - I was raised in an atheist household and we hardly ever talked about sex. I just felt dirty each time I came close to a screen where something sexually explicit was being shown. Even soft core (heck, only soft core - I never even encountered anything else until I started consciously looking for it).
When I was 13, I discovered that lots of people watch porn. It was a bit of a shock to me. I only found out because of a school trip wherein my roommates found HBO, which was apparently showing edgy stuff at night. Until then, porn was something for disturbed, messed-up perverts. I mostly stayed in my room to avoid the TV that trip.
When I was 15, I discovered that most men masturbate. This was an even bigger shock. I had never felt ANY urge to masturbate (I didn't even know how; later on, I looked it up on Wikipedia to find out). That year, though, not only did I hear guys talking about it pretty openly, but I was starting to really get into internet forums, and was finding out some pretty shocking things there. Weed, as it turned out, wasn't just for pothead bums. Masturbation, as it turned out, wasn't just for disgusting perverts. It took at least two years for me to get over all of this, and at the time I found it absolutely fascinating.
Around my 16th birthday I decided I might as well see what all the fuss was about, and for the first time watched some porn. My immediate reaction, as I remember it, was disappointment. I enjoyed it, but wasn't really sure what made it such a big deal. Not until April 19, 2008, when out of curiosity I pulled it for the first time. I was so ignorant of it I didn't even realize what physical effect this would create, and had some cleaning up to do afterward, if you get what I'm saying. I've been doing it on a regular basis ever since. The urge didn't appear until after it had been done once, which makes sense when you think about it - people aren't drug addicts before they've actually tried the drug. The kicker is, though, I've probably gotten significantly less horny since I started... I have no idea whether this is related, or just part of my natural growth.
Anyways, that's more than anyone ever wanted to know about me.