Great question! I research and study Medieval and Ancient coin production and economics. NSFW obviously.

The coins you are referring to are called Spintriae: a copper or brass coin usually produced in the 1st century AD with images of sexual acts on one side, and a Roman numeral (usually 1-16) on the reverse. There is only so much we know about these, but they are indeed very interesting bits of numismatic/exnomumia history. We have almost no primary sources regarding these tokens directly, so a lot of what we get is speculation.

Let me begin by clarifying a few points: These are not regarded as coins by modern numismatic scholars. We will dive into some reasons why we have this view later. These tokens were made in the 1st Century AD by an unknown issuer. The Roman Empire continued to make their currency, and these coins did not "flood" the empire over the span of a few decades. They are relatively rare and we have not found and hard evidence from the time period past the Spintriae themselves.

One important factor to look at is hoard evidence. Historical and numismatic scholars can draw many conclusions from a large deposit of coins found in a particular area: Possible circulation areas, the age of the coin, the intended use of the hoard, many ideas can be concluded by looking at the condition of the coins and the coins that make up the hoard as well. Hoards are one of the most important aspects of historical numismatic research.

It's important to state that there are no known hoard finds of these, past a supposed find in Pompeii in the 19th century. If these were truly used in brothels, one would expect to find a hoard in brothels within the area, but we have no such evidence and I have read such in any studies, and I would be glad to be wrong on this point.

I will dive into a few aspects of ancient economy. We should cover a few basics first: Money at this point in time was based on the metal in the coin itself, known as commodity money. In the Roman empire, it is extremely common to see brass, copper, bronze coinage spent with silver and gold.

The base unit of Roman coinage is the As. This is usually a copper or brass coin. The base silver coin is the denarius, worth 16 Asses. These tokens commonly being labeled 1-16 has led to the thought that they were used as a way to count Asses, possibly for calculations or breaking change. But why go through the trouble of making them out of the same material as asses, which are regardless made of the same material? The conclusion is that they serve a purpose other than money.

Coins (money) are an invention used to make transactions easier: Plato suggests the aspects of money as Durable, Portable, Divisible, and Intrisically Valuable, and this is especially true in the ancient world as well as today. They should facilitate trade rather than becoming as burdensome as the goods themselves. Pieces of metal fit all of these in ways most goods and commodities cannot: your grain will rot eventually, oil spoils, and your cloth will fray. Coins are a great cross section of all of Plato's points of money, being made of gold and silver, they are definitely intrisically valuable for use in smithing and rarity of the metal. Metal is durable, small coins are more portable than bulky goods, and can be divided physically (cutting coins) or changing into different denominations.

Keep in mind however, that the coin's metal content is directly tied to their value at this point in the Empire's history. A large copper coin is worth more than a smaller one, and so on. So again, why make these tokens in the first place when we aleady have established coinage that is sufficiently meeting the needs of the population?

Now, thats a lot for me to say these are extremely low value and likely were not regarded as coinage of the empire in their day. These are generally referred to as "tokens" by numismatic scholars.

Often we see tokens used in extremely small transactions. Spintriae are made from coppper or bronze, but Romans were accustomed to using copper and bronze as money already. While they may not have been as valuable as gold and silver, they were great for these smaller transactions. A silver Denarius was not needed for smaller "single item" transactions; compare it to buying a stick of gum with a $100 bill.

We see this spring up a few times in history as well: The Winetavern Street pub tokens from the medieval times were pewter tokens likely used to track transactions for small purchases, assumingly a single cup of ale or wine, up until the introduction of a coin small enough to handle the transaction. So to view these as tokens is probably correct, because their value was likely low enough for a smaller transaction.

One important factor I also want to stress is the idea of coins as propaganda. The empire was huge, and not every citizen in the empire, let alone Italy, knew what the current ruler looked like. Julius Caesar made it legal to issue coinage bearing the likeness of a living individual, a large break in tradition from previous Roman issues that often depicted their gods or ideals. Scenes of victory paried with a likeness of the ruler allowed every person who handled coinage to know of the valor of the issuer as well as knowing what they looked like - an extremely important attribute in the buildup to the Crisis of the Third Century. At this point in history, the Roman Republic had changed into the Empire, and the idea of "Might is Right" becomes more common as means of passing power during this period. This is a very generic glossing over of 300 years, so please read up on this point in history if you are interested in it. But the important bit I want to get to is the fact Romans and citizens of the Empire attributed morals to their coinage as well. Gods, might, and even status of the issuer is discernable from the coin - image, quality of engraving, quality of the metal and imagery depicted on the coin all play a factor. We have a quote from the Greeks stating: "Whose image has this Sesterius? Trajan? Take it! Nero? Throw it out, it is bad, worthless!" We know that Nero's coinage was not supressed after his reign, so many numismatics conclude that this is a joke - attributing the morals of Nero to coins issued with his image. This is by no means evidence, so please take it with another grain of salt. I explain this to say that if Romans do attribute such morals to their coins, it makes sense to think they would have been used in a brothel or similar establishment. We have graffitti in Pompeii stating if one wishes to visit a specific prostitute: "he needs 16 asses". This number again comes up, but is it coincidence? Again, we don't know.

As I said earlier, we have very little primary sources of information past the Spintria themselves. all of this is just the current state of speculation regarding these tokens.

Some Spintria are said to have been found in Pompeii in the 18th century, and it is unknown at least in what I have come across that they were found in hoards able to give us any viable informaiton. Again, some grains of salt.

The thought for a long time was that they were ways to enter into the brothel itself - a sort of ticket into the establishment. We have no evidence of this, however and future scholars point this out.

Others assert that they were used to pay the prostitutes, which if we could see the spending pattern of these could lend some validity to the story, or if they were found in a large hoard in or near a possible brothel. Again, we have no hoard finds of these, and we would expect to find these in hoards treated as true coinage if that is the case. One can assume this implies they were spent in circulation, but again, it is unlikely due to how few tokens were produced. I have not come across anything that shows where specificly they were found in Pompeii.

Some suggest they depict the prices of the acts on the coin, but these are generally only pushed by "Pop History" websites. Again, no proof one way or the other.

There is a story of a man being killed for bringing the likeness of Caracalla into a brothel, the likeness being on his coins. This is dubious at best, but the idea gives us a great thought problem: If this were true how does one get around this rule? One can take their money and exchange it at another location and receive these tokens in return. They are not breaking the rule. I personally believe this is the most correct idea we have at the moment. The most interesting part is it now incentivized the person to spend their money by using the token economy rather than true money.

This is a prinicpal of psychology designed to separate you from your money, and economists have suggested the Locker Token idea to establish one possible use for Spintriae. Once the money is exchanged for tokens, the service may not seem as expensive because it cost say 5 tokens instead of 3 denarii. Think of public lockers used for temporary storage, they usually require a special token to gain access to the locker itself (thus, locker token). Once you purchase the token, it is worthless. You can't get your money back and there is no reason NOT to spend all of your tokens. When it comes time to buy another token, you will remember spending the worthless token to get something more recently and likely be more willing to part with money since that "pain" of spending money is further back in your memory. Same idea here: Someone purchases tokens to spend inside the brothel to avoid this alleged law against bringing in the likeness of the emperor.

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