Harems were common in the Orient; the Chinese emperor, the Ottoman sultan, and the Mongol emperor all had many wives.
Roman Emperors did not have a similar custom. Why is the harem common in the Orient, but not in the Western world? What institutions in the West fulfilled the role that the Harem played in the East?
One key lies in their treatment of illegitimacy, or bastardy. In Roman society, as is typical of the West in general, illegitimate children had no formal link to their fathers. This was true from the earliest times, and lasted well into the Imperial period before a softening of the laws occurred in the second and third centuries.1
In the Roman setting, illegitimacy was chiefly a disability in inheritance law. It was a major problem in cases of intestacy, where the father either left no will or his will failed for technical reasons ... in Roman law illegitimate children had no legal relationship to their fathers.2
Conversely, in the Eastern societies (at least, the named examples with royal harems), all sons were acknowledged and given a place in the succesion line.
Let us disregard the role of sexual jealousy and focus on the practical impact of such succession laws. Since any of them could produce the future emperor, all sexual partners of the ruler must be guarded from other men. This is to ensure that they could only be impregnated by the ruler himself, so that the correct progeny continue to sit on the throne.
Indeed, this is why harems were commonly staffed by eunuchs6:
In many of these cultures [practising castration] eunuchs were employed as harem attendants or guards. This was done to keep the attendants or guards from sexually interacting with the women or getting the women pregnant.7
The lack of formal rights to succession in the Roman system precluded the need for highly secluded, guarded harems in the style of the Orient. However, this does not mean Roman emperors behaved any differently in their sexuality.8 The Roman Empire was unabashedly a slave society. Slaves, being properties, were essentially at the mercy of their masters' whims. As is typically the case, slaves were commonly sexually exploited by their owners from at least the late Republican period.
In [the first half of the second century B.C.] promiscuous, sometimes brutal, sex indulgences with slave women becomes noticeable in the Roman historical sources. In the following century this freedom of sex indulgence with female slaves become more marked and is openly discussed by Horace.9
Naturally enough, the Roman Emperors were no exceptions, though they may have been under more pressure to keep their activities hidden from the Roman public. Tiberius for instance was said to have bought numerous prostitutes to the relatively remote island of Capri. Similarly, the historian Publius (or Gaius) Cornelius Tacitus reported that Nero tried to hide his night life of sexual debauchery.
[Petronius] described fully the prince's shameful excesses, with the names of his male and female companions and their novelties in debauchery, and sent the account under seal to Nero ... When Nero was in doubt how the ingenious varieties of his nightly revels became notorious, Silia came into his mind, who, as a senator's wife, was a conspicuous person, and who had been his chosen associate in all his profligacy. [Annales. 16. 19-20]
The luridly detailed and particularly depraved tales of orgies, incestuous rapes and virgin fetishism involving emperors reported by Suetonius may or may not be accurate. Yet the mere fact that such rumours were preserved indicates that contemporary Romans did not find them so implausible.10
References:
1. Schäfer, Peter. The Talmud Yerushalmi and Graeco-Roman Culture. No. 93. Mohr Siebeck, 2002. "[I]n the 2nd century, the birth of illegitimate children could be registered in the official records previously reserved for legitimate children, and by the early 3rd century Romans interpreted an earlier law in a manner that set aside the rights of patrons to inherit form their freedwomen's estates, in favor of illegitimate as well as legitimate children."
2. Phang, Sara Elise. The Marriage of Roman Soldiers (13 BC-AD 235): Law and Family in the Imperial Army. Vol. 24. Brill, 2001.
3. Hinsch, Bret. Women in Early Imperial China. Rowman & Littlefield Publishers, 2010. "Chinese marriage was theoretically monogamous ... An emperor might enjoy hundreds of consorts, but he had only one true wife."
4. Faruqui, Munis D. The Princes of the Mughal Empire, 1504–1719. Cambridge University Press, 2012. ""Like the Ottoman Empire, [the Safavid dynasty's] early succession practices reflected Turco-Mongol-inspired ideas that vested imperial sovereignty in all male members of the royal clan or family."
5. Rossabi, Morris. "Khubilai Khan and the women in his family". From Yuan to Modern China and Mongolia: The Writings of Morris Rossabi. Brill, 2014. ""Yet even the lowest concubines could rest assured that her children stood to inherit property from their father. They were not treated as illegitimate children without any claims to their father's wealth. Though the first wife was accorded a higher status than the rest and her children inherited a larger share of their father's property, the children of the other wives and concubines received a portion fo the legacy."
6. Smith, Bonnie G. The Oxford Encyclopedia of Women in World History: 4 Volume Set. Oxford University Press, 2008. "Eunuchs were, and are, most familiar from their role of attending upon and guarding women ... the appeal of eunuchs was that they could not impregnate the women they were charged with guarding."
7. Giles, James. "Sex hormones and sexual desire." Journal for the Theory of Social Behaviour 38.1 (2008): 45-66.
8. Veyne, Paul. The Roman Empire. Harvard University Press, 1997. "Emperors, even when married, kept a harem of slave concubines in the palace, and Claudius was known to sleep with more than one at a time."
9. Westermann, William Linn. The Slave Systems of Greek and Roman Antiquity. Vol. 40. American Philosophical Society, 1955.
10.
Icks, Martijn, and Eric Shiraev. Character Assassination Throughout the Ages. Palgrave Macmillan, 2014. "[T]he precondition for its publicity was the fact that it did not seem implausible to contemporaries: people apparently assumed that all kinds of things might have happened within the hidden confines of the imperial palace. Thus it did not seem at all absurd to Suetonius that Caligula had transformed his palace into a brothel and forced noble women and boys to prostitute themselves there."
The Romans had two institutions that the Eastern rulers lacked: monogamy, and primogeniture.
Monogamy meant that even wealthy Romans would have only one wife. One might have plenty of mistresses on the side, but these weren't "wives. Which led to the next thing, primogeniture. That is, inheritance of the family line by the one and only oldest son, begotten of one legitimate wife.
The eastern rulers on the other hand, practiced polygamy. That meant that the ruler married numerous women (some of whom he never slept with). But it also meant that any son of any wife had a "shot" at the throne, not just the first born son of one wife.
The whole point of the harem was not just to provide a plentiful supply of sex partners for the ruler, but to deny them to other men. This was necessary so that claimants to the throne would all have royal blood (in the days before DNA tests). On the other hand, Roman rulers did not have to deny their mistresses to other men; many of those mistresses were actually other men's wives.
While the Roman emperors were no doubt a happy bunch, they certainly weren't homosexual.
They had wives, children, extra-marital affairs (often to the horror of the court), concubines, the works.
But what they didn't have was a big building full of sex slaves, gathered as loot during military campaigns and taken into their household in exchange for favours from foreign dignitaries.
Such wouldn't do in Roman society where family values were held in great esteem (at least in public).
I would argue they most likely did.
In Suetonius' book, The Twelve Caesars, the Roman biographer criticizes Domitian for allowing his concubines to comb his hair, act his servants, etc. Suetonius wasn't criticizing him for having concubines or for letting woman do his work, he was criticizing Domitian because he did it for sexual pleasure.
To be fair, I don't know Latin, so it's possible that the translator might not have translated the term "concubine" correctly. And to add to that, Suetonius also scorned Claudius (or maybe another emperor, I can't remember which exactly) for proposing to allow Emperors to marry multiple wives, which implies that it wasn't allowed at the time.
So make your own conclusions.
Another thing that makes a harem less necessary: divorce
Other answers already discussed that the concept of monogamy was essential to Roman culture, at least in the sense that one could only have one legal husband/wife at a time.
However, if the emperors eye falls upon a new woman, in a harem-culture the emperor would simply add this new woman to his harem. In contrast in ancient Rome, the emperor could simply divorce his current wife and marry the new woman. Hence, no need for a harem.
Wikipedia lists divorces for Caesar and enter link description here.
