פירוש על קידושין 44:18

The initial assumption is that women do not enjoy unnatural (anal) intercourse. Therefore, he could acquire her through such a form of intercourse, because she would be doing this just as an act of servitude (again, this is not sanctioned—the Talmud is asking how come this does not work).
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.