In the cultures of Greece and Rome, attitudes to sex differed from time to time, and also from place to place. In the first instance it must be remembered that Greece was originally composed of city states, each with its own set of laws, and like all individual societies, with its own set of mores and morals. Rome was not much different at the beginning, when various tribes inhabited the Italian peninsula, and which only much later, probably during the eighth or ninth century BCE started to form the nucleus of what was to become the queen of the mediterranean world, Rome, and even later still, the capital of a vast empire.
Furthermore, attitudes toward sex and sexuality would also have differed from one class to another, the nobility not necessarily sharing the morals of the lower classes in a world where everything was structured around the rigid seperation of the classes. It was only in 445 BCE that a lex Canuleia abolished the prohibition on patricians marrying plebeians in Rome, at the same time instituting other marital prohibitions based on class differentiation. The fact that legislation regulated these prohibitions must mean that the current morality and sexuality at the time demanded such measures. A certain type of marriage, confarreatio, a highly formal and ritualistic procedure during which the spouses exchanged sacred spelt cakes, was concluded only between persons of patrician class, and was not available to the lesser classes.
At the same time, the attitude to marriage in Rome was not so puritan that bigamy was prohibited. It merely gave rise to infamia attaching itself to the person engaging in such conduct, which entailed a loss of civic standing in the eyes of the law, which precluded the individual from participating in certain functions or rituals of society for as long as the situation obtained.
Necessarily in any society where slavery was one of the foundations on which the entire system functioned, the status of a person as free or in bondage would also have had some effect on how sexuality and sexual relations were viewed. As can be gathered from Roman poetry and prose (especially the erotic poetry of Catullus and prose such as Petronius’ Satyricon), there were certainly some classes who viewed sexuality and sex with a more relaxed attitude than others. It would seem that there was no distinction drawn on a rigid basis between homosexuality and heterosexuality. The distinction that did matter, and this it seems is true of both Greece and Rome, was the function performed during sexual intercourse, being either passive (receiving) or active (penetrating). Thus it was the function of the male to penetrate, and in this regard it was not regarded as improper for unmarried men to have sexual intercourse with other men, provided that the passive partner was not a free born or freedman. It was regarded as being unworthy of a man to be the passive partner, unless he was a slave or perhaps even a non-Roman or non-Greek. It was not unusual for boys during puberty to be presented with a slave which had to see to the sexual needs of his master, obviously being at the receiving end. Upon marriage (or earlier, if the master succeeded in obtaining the necessary relief from a female lover or even prostitutes), all such relations would cease. Similarly it was the function of the woman to be the passive partner in the act.
Adultery was severly frowned upon (while it nevertheless remained, as it still does, a remarkably popular pastime), especially when “committed” by women. Compare for instance the famous story of Caesar’s wife whom he divorced simply as a result of the rumour of her infidelity, while Caesar by several accounts was always up for it, regardless of whether the other was male or female. It is more than probable that the attitude was far more relaxed where men were concerned. For this reason (and also due to various other considerations of the Roman law of persons and succession which are not important in this discussion) Roman law developed an extensive system of adoption, in order to allow especially sons from even loose liasons to be adopted into the father’s family, usually when there were no legitimate male heirs (only men could inherit, the women falling under the patriapotestas of the family head, who was responsible for their wellbeing and care).
So, in the ancient world, sexuality was defined more by virtue of one’s status, than by the choice of the sex of the partner, provided that where the partner in the case of men, was also male, it was more than preferable that he be in bondage, or at the very least, a non-citizen. Of female sexuality, less is known, because writing was the province of men, and besides, it was not very manly to involve yourself with what women did in their quarters. It seems that in Rome at least, lesbian relationships, while hardly approved of, were not as rigidly discounted as relationships between men, and therefore seemingly such relationships were accepted to a point.
Further reading: The Oxford Companion to Classical Civilization.