There is a myth among some people, including scholars, that pornographic images began with the invention of the photographic process.
It is a way, I suppose, to keep us focused on the idea that today's immorality is far worse than yesterday's. But that all sexual art before the time of Christ represents a sacred union is, nevertheless, a lie.
Of course, people have been prevented from knowing about these images by virtue of our puritanical outlook on things. The repositories for artifacts here in California contain more than a few examples of immaculately carved stone penises, the "sacred pestles" as would be explained to you if you were to wrangle permission to actually see one. American Indians didn't have a sense of humor? A sense of the erotic? Not even when rhythmically pounding acorns with a blunt cylinder?
The sacred is the way we explain away much of what we can't--or don't want to--understand as erotica or porn. It's a smokescreen at best. Once Greeks and Romans learned how to make cheap, molded oil lamps, the market in ports like Ostia were flooded with sexual imagery in three dimensions for sailors with an itch. Call it sacred if you wish. Whatever lights your wick. But as with the image above, reproduce the same thing with a couple of models and a camera and it'll be called porn by just about anyone.
Would it be beyond the realm of modern thought to believe that through sexual or even pornographic images, cultures have found a way to mitigate the sexual tensions that limiting access to sex imposes? Is providing a safe context for sexual imaginings as wrong and dangerous as it is universal? You decide.
The secret room or "Camera Segreta" in the Naples Archaeological Museum, containing sexual artifacts from excavations at Pompeii and Herculaneum, has been closed to all but the very rich and influential for 200 years. But despite vociferous protest, it is now open to the public. You do have to go through a few hoops to get in. After you buy your general admission ticket, you might have to ask about the secret room (camera segretta). You'll be directed to another kiosk where you'll be assigned a visitation time and given a voucher good for that time period. All tours are free and guided by someone who speaks your language but who isn't necessarily knowledgeable about the artifacts. You can hire an archaeologist to tell you more about them if you wish.
The interesting thing is, you are free to take pictures--as long as you don't use flash.
The picture above is the type of fresco many of the higher Roman class would have lining their bedroom walls. The same type of thing might appear in a brothel. And goat sex, well, that's another thing...
Priapus is a frequent visitor to the bedroom. He's the country version of a beloved but minor fertility god, "protector of livestock, fruit plants, gardens and male genitalia" according to Wikipedia, and obviously he's taken care of his.
If you're interested in more of the erotic images at Pompeii, this book may do it. Featuring over 80 erotic works of art and written by an authority on Pompeii, Eros in Pompeii will give you a clear understanding of how the erotic was a feature of Roman life, and not just brothel decorations. By the way, the erect penis figure was nothing more than a good luck charm to the Romans.
More on Ancient Erotica, Pornography, and Sexual Customs
More Suggested reading on ancient erotica and sexual attitudes.