What's with Pan's (the goat~man) Junk?

Pan is notorious for his sexual powers, and is often depicted with an exaggerated penis (phallus). Such symbols often represent the fertility and cultural implications that are associated with the male sexual organ, as well as the male orgasm.

Pan and Bacus was a mythical character and therefore the penis is a representation of how the artists pictured it in. There is no historical basis for it since it is not an actual historical figure

Erotic art in Pompeii and Herculaneum was discovered in the ancient cities around the bay of Naples (particularly of Pompeii and Herculaneum) after extensive excavations began in the 18th century. The city was found to be full of erotic art and frescoes, symbols, and inscriptions regarded by its excavators as pornographic. Even many recovered household items had a sexual theme.

The ubiquity of such imagery and items indicates that the sexual mores of the ancient Roman culture of the time were much more liberal than most present-day cultures, although much of what might seem to us to be erotic imagery (e.g. oversized phalluses) could arguably be fertility-imagery. This clash of cultures led to an unknown number of discoveries being hidden away again. For example, a wall fresco which depicted Priapus, the ancient god of sex and fertility, with his extremely enlarged penis, was covered with plaster (and, as Schefold explains (p. 134), even the older reproduction below was locked away "out of prudishness" and only opened on request) and only rediscovered in 1998 due to rainfall.

In 1819, when King Francis I of Naples visited the Pompeii exhibition at the National Museum with his wife and daughter, he was so embarrassed by the erotic artwork that he decided to have it locked away in a secret cabinet, accessible only to "people of mature age and respected morals". Re-opened, closed, re-opened again and then closed again for nearly 100 years, it was briefly made accessible again at the end of the 1960s (the time of the sexual revolution) and was finally re-opened for viewing in 2000. Minors are still only allowed entry to the once secret cabinet in the presence of a guardian or with written permission.