Since feelyunicorn mentioned his introduction to porn... I figured I would too
First, a bit of backstory. For my 10th birthday, my parents got me only books, and lots of them. From that point on, I read a lot and often, although they say they got me books because I liked to read so I must have started earlier than that.
The point is, I was following a bunch of collections. Things in the veins of Goosebumps, although not this specific one, none where horror. One was humour, one was medical, one was romance...
And that's the one that interest us here.
I was on vacation with my family when I was 12 or so, maybe even a bit younger. We were in a vacation club, where you'd rent a house, and there was a center in the middle with stores, etc. I would often go there on my own, go to a bookstore and buy a bunch of books. Once I did that, and decided to pick up some more romance books.
They didn't have the series I was following ("Grenadine Heart". Stories of girls around my age having school romances) but they had a lot of romance books and so I picked one up.
It turned out to have an erotic scene in it, although for the most part it was plain romance. It was my first experience actually reading about a sex scene. I went back to the store and bought some more after I was done that one.
I'm still a bit curious. I was obviously a kid, and the clerk was selling me graphical books. He didn't seem to care. I'm also didn't hide the books when reading them so my parents must have known what I was reading. Did everybody assume the romance had no sex? (I guess it's not a stretch, I did assume that, but I was young and they were adults. And it was like the most famous erotic books publisher.)
Anyways, that's for written material. As far as visual material goes, I once went to a political thing with some other students when I was in high school, and it was two days long so we had hotel rooms. I stayed with another female who was older than me (I must have been 16 and 17) and who watched porn in the hotel room. I remember looking at it from the top bunk bed, and being aroused but a bit turned off at the same time (not enough affection going on, and it kept switching scenes and having different people having sex, I couldn't follow the story).