You can see the graffiti yourself at
http://ancientgraffiti.org . Cursory inspection revealed zero referring to people relieving themselves in the street, though I don't doubt those exist. Isolated names are very frequent.
Some of these are pretty interesting:
> IV Non(as) Iul(ias) paenulam, palliolum [?posita ad Fau]stilla(m) Pr[o---] L usur[a] [deduxit?] XIIIS (?) [aeri]s a(sses) VIII
> Four days before the Nones of July, a traveler's cloak and a small cloak (were deposited with Fau)stilla (for collateral). Fo(r) fifty (denarii?) she [is charging] 13.5 (coppe)r a(sses) (per month, as interest). 8.
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> ABCDEFGḤỊKOPQRTVX
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> ABCDEFGHLIMPKONQRXTVS
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> ABCDEFGHI (gap) KL (gap) MNOPQRSTVX
This one is an exact mirror of the modern alphabet (except Y and Z); I'm not sure why there are gaps.
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Some are timeless:
> Victor cum Attine hic fuit
> Victor was with Attine here.
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> Hic ego puellas multas futui
> I fucked a lot of girls here.
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> Fututa sum hic
> I [a woman] was fucked here.
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> Antiochus hic mansit cum sua cithera
> Antiochus stayed here with his cythera [not sure what this means]