This is a rather broad question which will, unfortunately, have a very broad answer.

In 'short', the Roman and Greek social systems of thought conceived of the erotic very differently than we do. The mosaics in Pompeii weren't pornographic to the Roman mind, neither was statuary of Priapus, Catullus on the other hand... well, he certainly did use some choice language in some of his poetry (I'm looking at you, Carmen 16).

Eroticism was simply a fact of life back then. Mosaics, while they may have been somewhat prurient in nature were anything but pornographic for the most part. Arguably the most erotic pieces of surviving art from the Roman era comes from the Lupernar, a massive brothel in Pompeii. These paintings depict a number of explicit sexual acts and a working theory for scholars in the field of Ancient prostitution, such as McGinn, believes they served as a kind of menu for patrons. Furthermore, many of the similarly "pornographic" works of art from the period are often relegated to more private chambers rather than public spaces.

That said, there are many things that were displayed in public places which, again, would be censored by modern standards. Indeed they still are, there is a room at the Museum of Naples which you have to be 18 or older to enter because of the artifacts and artwork at Pompeii, and some of the restoration work done at Pompeii revolves around removing the whitewash that archaeologists used to cover up artwork that clashed with Victorian sensibilities.

Of the physical goods (statuary etc.) it is important to understand why they made this kind of artwork. For the most part it was for some kind of protection. Depictions of the genitals was often done as part of an apotropaic (Greek roots, lit. Turn away) tradition which would turn away evil spirits. You can find similar occurrences of this type of magic in many cultures around the world, such as Bhutan where it is a common practice to paint phalli on doors and walls to drive away evil spirits. In Rome phalli had similar ritualistic functionality, as well as the humorously practical application of driving away would be thieves as we are told in the Priapeia. They certainly wouldn't want to be on the receiving end of a Priapus scorned. One last note on nudity in statuary (and often mosaics and paintings) there is something called heroic nudity. Nudity which invokes heroic images, such as Hercules who is often nude save for his club and lion-skin cloak.

Finally, Catullus. It is important to recognize that he is not even attempting to write in a high poetic register as say Ennius before him or how Virgil would write after. He captures some aspects of the vulgar language spoken at the time. In that regard he is very similar to Martial in his epigrams (my personal favorite example of erotic literature). While we obviously don't have recordings of every day life, it is widely agreed, on account of features such as its lexical concurrence with personal letters and graffiti, that some of this language was used in common speech. e.g. futuo, irrumo, etc.

We can say that all of these were common features of every day life. An average Roman would have been quite familiar with many of these things whether it be from a private garden, or bed chamber, or the walls of a brothel, or carved into the streets. It was just a part of every day life up to actual sex (about which Romans tended to be very private about. Sorry folks, wild Roman orgies weren't really a thing!) Everything short of that, including talking about sex, was totally fair game.

I hope this answered some of your questions. If you have more specific questions I would be happy to attempt to answer any follow-ups.

Here are some basic sources / further reading:

Adams, J. N. The Latin Sexual Vocabulary 1982 - requires a bit of Latin/Greek know-how

Hallett, J. P. & Skinner, M. B. Roman Sexualities 1997

Larson, J. Greek and Roman Sexualities: A Sourcebook 2012 - lots of literary discussion with passages

McGinn, T. Prostitution, Sexuality, and the Law in Ancient Rome 2003

Skinner, M. B. Sexuality in Greek and Roman Culture 2012