Aristotle, Problemata 4.26

Why do some get pleasure while being passive in Aphrodisian activities, and (of these) some while at the same time playing the active part and others not doing so?

Is it that for each surplus there is a place into which it is naturally separated, and upon exertion emergent pressurized air makes (the place) swell up, and helps secrete it (i.e. the surplus), for example urine into the bladder, food with the moisture extracted into the intestines, tears into the eyes, mucous into the nostrils, blood into the veins, and then similarly to these also the generative fluid [gonee] into the testicles and penis. In the case of those indeed who have the conduits not according to nature, but either because those (conduits leading) into the penis have been blinded -- as is characteristic of eunuchs and eunuch-like persons -- or otherwise too, such moisture flows into the rectum. For it also passes through this (i.e. the rectum). The proof (of that) is the contracting of this place in intercourse and the oozing of the (places) around the rectum. Therefore if someone is highly sexual, it gathers here in these (places), so that whenever desire [epithumia] is generated, this (place) in which it (i.e. the fluid) is collected desires friction. And the desire is generated both from foods and from thought. And when it (i.e. the pressurized air) is stirred up by anything, the pressurized air concentrates here, and this residue flows where it naturally does.

And if it (i.e. the residue) is thin or mostly air when this (i.e. the pressurized air) goes out, the erections -- like those of boys and those in the prime of puberty -- sometimes cease without any fluid coming out, since the fluid is dried up. If neither of these two things occurs (i.e. fluid flowing to penis, testicles or rectum, or else drying up), he keeps desiring until some one of these things happens.

Those who by nature are like females [fusei theeludriai] are put together so that nothing or little comes out in the place where it comes out for those who have them according to nature, but rather (it comes) into this place. The reason is that they are put together contrary to nature. While being of male sex, they are so disposed that by necessity this place of them has been made imperfect. One (kind of) imperfection makes a complete annihilation, while another (kind makes) a distortion. But it is not in fact the former (i.e. the complete annihilation of the male parts) -- for he would have become a woman. Hence it (i.e. the pressurized air) must be diverted and must impel upon the generative secretion somewhere else. For this reason, they are also insatiate, like women. For the moisture is little, and is not forced to go out, and quickly cools. And those (who have it) near the rectum desire to be passive, and those (who have it) in both places desire to be active and passive. Wherever most of it is, that is what they desire more.

To some this affliction occurs because of habits. As often as they do it, they get pleasure and they emit their generative fluid [gonee] this way. Therefore they desire to do (it) in the places that result in these things, and more and more habit becomes just like nature. For this reason, those who make a habit of being passive in the Aphrodisian activities -- not before puberty but about the time of puberty -- because the memory comes to them when they are in need (i.e. horny), and pleasure accompanies the memory, because of habit they desire to be passive as if it were natural to them. Indeed, with regard to many things, habit becomes like nature. If by chance they are lustful and soft, all of these things are more likely to occur.