"In Aristophanes' story of the origins of men and women from two aboriginal, globular creatures who had either two male organs, two female organs, or one of each, only those who descended from the hermaphroditic form would "naturally" seek the "opposite" sex in order to achieve union. Otherwise, as Aristotle pointed out in the context of "what is natural is pleasant": like loves like, jackdaw loves jackdaw. In fact, reproductive heterosexual intercourse seems an afterthought. [...] "Love is born into every human being" the story concludes [...] But what we would call the sex of that human being seems of only secondary importance.
But where honor and status are at stake, desire for the same sex is regarded as perverse, diseased, and wholly disgusting. A great deal more was written about same-sex love between men than between women because the immediate social and political consequences of sex between men was potentially so much greater. Relatively little was directly at stake in sex between women. Yet whether between men or between women, the issue is not the identity of sex but the difference in status between partners and precisely what is done to whom. The active male, the one who penetrates in anal intercourse, or the passive female, the one who is rubbed against, did not threathen the social order. It was the weak, womanly male partner who was deeply flawed, medically and morally. His very counternance proclaimed his nature: pathieus, the one being penetrated; cinaedus, the one who engages in unnatural lust; mollis, the passive, effeminate one. Conversely, it was the tribade, the woman playing the role of the man, who was condemned and who, like the mollis, was said to be the victim of a wicked imagination as well as an excess and misdirection of semen. The actions of the mollis and the tribade was thus unnatural not because they violated natural heterosexuality but because they played out -- literally embodied -- radical, culturally unacceptable reversals of power and prestige.
Similarly, when power did not matter or when a utopian sharing of political responsibility between men and women is being imagined, their respective sexual and reproductive behavior is stripped of meaning as well. Aristotle, who was immensely concerned about the sex of free men and women, recognized no sex among slaves. A "woman," as Vicky Spellman puts it, "is a female who is free; a 'man' is a citizen; a slave is a being whose sexual identity does not matter. For Aristotle, in other words, slaves are without sex because their gender does not matter politically."
-- Thomas Laqueur (1992) Making Sex: body and gender from the Greeks to Freud, pp. 52-54 (emphasis mine)
Note that "sexual identity" here means gender and indeed all uses of the word "sex" in this text refer to it in the sense of biological sex.