Plato's Symposium:

Each of us when separated, having one side only, like a flat fish, is but the indenture of man, and he is always looking for his other half. Men who are a section of that double nature which was once called androgynous are lovers of women; adulterers are generally of this breed, and also adulterous women who lust after men, but have female attachments; the female companions are of this sort. But they who are a section of the male follow the male, and while they are young, being slices of the original man, they hang about with men and embrace them, and they are themselves the best of boys and youths, because they have the most manly nature. Some indeed assert that they are shameless, but this is not true, for they do not act thus from any want of shame, but because they are valiant and manly, and have a manly countenance, and they embrace that which is like them. And these when they grow up become our statesmen, and these only, which is a great proof of the truth of what I am saying. When they reach manhood they are lovers of youth, and are not naturally inclined to marry or to begat children--if at all, they do so only in obedience to the law; but they are satisfied if they may be allowed to live with one another unwedded; and such a nature is prone to love and ready to return love, always embracing that which is akin to him. And when one of them meets his other half, the actual half of himself, whether he be a lover of youth or a lover of another sort, the pair are lost in an amazement of love and friendship and intimacy, and one will not be out of the other's sight, as I may say, even for a moment: these are the people who pass their whole lives together, yet they could not explain what they desire of one another. For the intesne yearning which each of them has towards the other does not appear to be the desire of lover's intercourse, but of something else which the soul of either evidently desires and cannot tell, and of which she has only a dark and doubtful presentiment.