[Translated by W.S. Hett,  Problems, Cambridge, MA, 1970. Translation amended by me.]

The text is pretty hard for a modern person to follow, because it is full of medical assumptions that we no longer believe. I will try to sum these beliefs up as well as I can.
It seems that, in Aristotle's world, semen was considered a waste product, like blood, feces and other excretions of the body. Whereas blood flows into the veins and feces to the rectum, semen flows to the penis and testicles, where it accumulates. During the sex act, it is cooked into shape by the heat of male lust, and then it waits for a blast of air -- a breath -- from the brain to push it out in an ejaculation. To Aristotle's mind, women and passive men do not have ejaculations -- they only get a little wet from the flow of their sexual fluids. Never having observed a passive man's ejaculation perhaps, Aristotle is able to assume that passive men's moisture, which would ordinarily flow to the penis, is blocked and instead flows to the buttocks, which causes them to desire "friction" in that place.
 

Problemata, IV 26

Why do some men enjoy sexual intercourse when they play an active part and some when they do not (i.e. when they play a passive part)? Is it because for each waste product there is a place into which it naturally secretes, and, when energy is employed, the breath, as it passes out, causes swelling, and expels the waste product; for instance urine passes into the bladder, and desiccated food into the stomach, tears into the eyes, mucus into the nostrils, and blood into the veins? Just in the same way semen passes into the testicles and privates. In those whose passages are not in a natural condition, but either because those leading to the testicles are blocked, as occurs with eunuchs and in other impotent persons, or for some other reason, such moisture flows into the buttocks; for it passes in this direction. This is proved by the contraction of this part in intercourse and the wasting of the parts about the buttocks. If then a man indulges in sexual intercourse to excess, the semen collects in these parts, so that when desire comes, then that part desires friction in which it is collected. The desire arises partly from food and partly from imagination. For when he is excited by anything, the breath races to that part and such waste product flows into its natural place. If, then, it is light or full of breath, when this passes out, the tension ceases without the emission of moisture, as it does sometimes both with boys and adults. (This also occurs) when the moisture dries up. If, however, neither of these results occurs, desire continues until one of them happens. But the naturally effeminate are so circumstanced that little or no secretion occurs in the place in which it occurs with normal persons, but it is secreted in this region. The reason is that such persons are unnaturally constituted; for though they are male this part of them has become maimed. Such maiming produces either complete destruction or a distortion of type. The former is impossible in their case for it would imply their becoming female. So it must involve distortion and an impulse in some direction other than the discharge of semen. So they are unsatisfied, like women; for the moisture is slight and does not force an exit and is quickly chilled. Those with whom the semen travels to the buttocks desire to be passive, and those with whom it settles in both places desire to be both active and passive in sexual intercourse; in whichever direction it inclines the more, so do their desires. In some cases this state is the result of habit. For men are accustomed to enjoy what they normally do, and to emit semen accordingly. So they desire to do that by which this may occur and so habit tends to become second nature. For this reason those who have not been accustomed to submit to sexual intercourse before puberty but at about that time, because they have recollection of their enjoyment and pleasure is associated with the recollection, because of their habit desire the passive state, as if it were natural, numerous occasions and habit having the same effect as nature. If a man happens to be both lustful and effeminate, this is all the more likely to occur.