To be begin with, unlike many other cultures of antiquity, the Romans (and the Greeks who they morally emulated) were monogamous. Roman men had one wife at a time with no concubines. Divorce was not uncommon but required a cause such as infertility on the part of the woman or abuse and neglect on the part of the man. Many prominent Roman men did have many wives due to divorce and remarriage, just not at the same time. Some Roman men had mistresses but this was frowned upon.

Adultery by the woman was also cause for divorce and during the reign of Augustus was punishable by banishment after he proclaimed the Lex Iulia de Adulterous Coercendis, a law intended to protect traditional Roman family values. Caesar was in fact a divorcee, he divorced his second wife Pompeia after a public scandal. A man called Clodius disguised hinself as a woman entered the women-only festival Bena Dear with the express intent of seducing her. He didn't succeed but Caesar divorced her anyways, saying "my wife ought not even to be under suspicion". This proves that Caesar was indeed a very image conscious and scandal averse man who is keen to appear the very definition of the virtuous patrician.

1. We know Julius Caesar has one illegitimate son with Cleopatra, Caesarion, who became the last Pharoah of Egypt after his mother's suicide. He never formally acknowledged the boy as his son but it was an open secret and he was said to have borne a strong resemblance to Julius. Octavian would later have him killed when Caesarion was 17 to eliminate a potential rival as Caesar's heir. He acted on advice from the philosopher Arius Didymus who said "Too many Caesars is not good".

2. The Romans had laws against adultery and rape but those were limited to freeborn Roman women. When it came to slaves and barbarians they could, and did, rape with impunity. It is a well known fact that Roman legions did rape the people they conquered. And the people they raped was not limited to women and girls, they often raped boys as well.

We do not know about Caesar's personal conduct in these matters, for he would never mention such a thing himself nor would any later historians, but rape was considered a natural part of warfare and a way to punish the enemy. So if he did rape some women during war nobody would have batted an eye but he also wouldn't have acknowledged or even been aware of any children begotten that way.

Livy said of soldiers in the Carthaginian town of Locri during the First Punic War, "They defile matrons, maidens and free-born boys". Tacitus describes the rape of Batavia boys as a reason for the Revolt of the Batavians in 69 CE, during the Year of Four Emperors in the aftermath of the fall of the Julio-Claudian dynasty. He writes: "young, good-looking lads (for children are normally quite tall among Batavians) were dragged off to gratify their lust."

Same-sex sexual relations was quite normal in ancient Rome and was not shameful as long you are the active rather than passive party. There were persistent rumours during Julius' lifetime from his enemies such as Cicero and Bibulus that Julius was in a long-term passive homosexual relationship with King Nicomedes of Bythnia and would mock Julius Caesar by calling him the "Queen of Bythnia". The rumours were so oft repeated that Caesar felt the need to deny it under oath. The historian Suetonius would later write, "Caesar may have conquered the Gauls, but Nicomedes conquered Caesar". There may have been some truth in that and partially why Caesar didn't have more children.

3. There was a famous unidentified plant in the ancient world known for its contraceptive and abortative properties called the silphium. It was harvested from the coast of Cyrenaica (now Libya) in North Africa, namely the city of Cyrene. The plant was highly lauded for its contraceptive properties at the time but mostly went extinct around the 2nd or 3rd century BCE before the time of Caesar in the 1st century BCE. The most common theory for extinction was a combination of overgrazing by animals who were fed the plant to improve the flavour of their meat and overharvesting by humans. The Greek philosopher Theophrastus said the plant could not be successfully farmed. The last mention of such a plant was by Pliny, who said the last known specimen was given to Nero around 54 CE.

By the late Republic and early Emoure period silphium was longer in play but there were still other contraceptive or abortative herbs like pennyroyal (glechium). Both silphium and glechium would have been made into drinks. The herbs were said to stop menstruation or to end a pregnancy.

But outside of all this, low birth rate was a recognized problem in the late Republic and early Empire period. Augustus passed the Lex Iulius de Maritandis Ordinibus in 18 BCE which required all citizens to marry. He followed this up with the Lex Papua Poppaea in 9 CE which banned both adultery and celibacy (in this case meaning not being married) in addition to forbidding the marriage between freed slaves and members of the senatorial class. Widows and divorced women who were under fifty were compelled to remarry within a certain time frame.

It seems many Romans wanted children but could not have children, especially within the urban population. It's hard to say anything conclusively about rural peasants but since the empire's population did not significantly decline, perhaps this trend of low fertility did not apply to them. One theory says this was because of Roman bathing habits which were very hot, described as nearly scalding, and very frequent and very popular for all classes (Devine, 1985). Constant exposure to high temperatures can indeed be bad for sperm count.

Another complementary theory raising points to the fact that the cost of children in Rome during the late Republic was remarkably high for the ancient world and comparable to present day Europe. The senator and two-time consul Lucius Aemilius during the 2nd century BCE famously gave two of his four sons up for adoption because he could not afford to raise all of them and he knew many other families had no sons of their own. Children, both boys and girls, of senatorial class went to school and that was not free. Wealthier Romans hired Greek slaves as personal tutors for their children and the most learned slaves could be incredibly expensive.

Sources Cited: Discorides. De Materia Medica. C. 50-70.

Devine, A.M. The Low Birth-Rate in Ancient Rome: A Possible Contributing Factor. Rheinisches Museum für Philologist. J D. Sauerländers Verlag, 1985.

Kiefer, O. Sexual Life in Ancient Time. Kirk Press, 2011.

Mark, J. Love, Sex, Marriage in Ancient Rome. Worldhistory.org, 2020.

Suetonius, Julius. 49.

Tacitus. Histories. Book V Chapters 14-26. C. 100-110.

Riddle, J. "Eve's Herbs: A History of Contraception and Abortion in the West*. Harvard University Press, 1997.