By Torah law, idolaters do not have the strict ritual impurity of the Jews. However, in order to distance Jewish children from the children of idolaters, the Sages decreed to treat all idolaters and idolatresses as ritually impure because of zivah, as we have seen before. This was not, however, a real impurity, and if holy foods came in contact with such decreed impurity, they could not be burned – since in general it is forbidden to destroy sacrificial foods.
Therefore, the Sages now had to make a clear distinction between the ritual impurity of idolaters, which was only a decree, and that of Jews, which was the Torah law. They did it by declaring only some of the discharges of idolaters impure, but not all of them, so that it would serve as a reminder. Beit Shammai say that the uterine blood of an idolatress is pure, whether moist or dry. However, Beit Hillel gave it the impurity of other secretions, such as saliva and urine, which is impure only when moist but not when dry.
The semen of a Jew is ritually impure, and if it comes out of an idolatress, it makes her impure. The semen of a non-Jew is pure, and if it comes out of a Jewish woman, it does not make her impure. Rav Pappa asked the question about the semen of a Jew inside of an idolatress. But we just answered it!? He meant, after three days. We know that inside a Jewish woman it putrefies in three days and is thus pure, perhaps because she is always worried about mitzvot, which adds internal heat. This is not true about an idolatress. On the other hand, an idolatress eats various crawling creatures, and maybe that too adds heat? The question remained unresolved.
Art: Edward R. King - London Street Children