Consider my mind just blown, pun not intended:
[These coins] were used in ancient Rome to request and pay for different “services” in brothels and from prostitutes on the street. Since there were a lot of foreigners coming to the city that did not speak the language and most of the prostitutes were slaves captured from other places the coins made the transactions easy and efficient. One side of these coins showed what the buyer wanted and the other showed the amount of money to be paid for the act.
It’s thought that these coins were only produced for a short-time in the first century A.D. Among the acts depicted include blow jobs, gay sex, anal and other erotic acts.