This is, apparently, not a joke. Recall that these must all have been written before AD 79, when Pompeii was covered by molten rock from Mt. Vesuvius. It is simply humanity doing what we do. Personal favorites include:
II.2.3 (Bar of Athictus; right of the door); 8442: I screwed the barmaid
and
If anyone sits here, let him read this first of all: if anyone wants a screw, he should look for Attice; she costs 4 sestertii.
I could go on, but I’ll leave the rest for you.