Commentary for Kiddushin 44:18
שלא כדרכה מאי איכא למימר אמר רב אחיי [בר אדא] דמן אחא מאן לימא לן דלאו הנאה אית להו לתרוייהו ועוד משכבי אשה כתיב הקישה הכתוב כדרכה לשלא כדרכה
Our Rabbis taught: How [is a heathen slave acquired] by hazakah? If he unlooses his shoes for him [the purchaser], or carries his baggage after him to the baths; if he undresses, washes him, anoints,<span class="x" onmousemove="('comment',' Massaging with oil was an essential part of the bath. It was and is common in the Orient, and amongst the Romans and Greeks, and had its cause in the hot climate, which causes all living bodies to emit an unpleasant odour; v. Krauss, T.A., I, 229 and 233.');"><sup>31</sup></span> scrapes,<span class="x" onmousemove="('comment',' With a kind of brush to tone up the circulation.');"><sup>32</sup></span>
Daf Shevui to Kiddushin
There are two answers: 1) Some women do enjoy anal intercourse. 2) The Torah uses the plural in reference to forms of intercourse, thereby implying that they are all treated the same. Just as a female slave cannot be acquired through natural intercourse, she also cannot be acquired through unnatural intercourse.