A regular Cohein may not marry an eilonith (a woman who cannot bear) unless he has a wife and children. [The same is true of an Israelite. "Cohein" is stated only because of the demurral of R. Yehudah, who says that a Cohein, though he has a wife and children may not marry an eilonith, as opposed to an Israelite. The halachah is not in accordance with R. Yehudah.] R. Yehudah says: Even though he has a wife and children, he may not marry an eilonith, for she is the "zonah" referred to in the Torah (Leviticus 21:7). And the sages say that "zonah" is a proselyte, a freed maidservant, and a woman who had been lived with wantonly, [such as those interdicted by negative commandment or positive commandment, and, it goes without saying, those liable to kareth or judicial death penalty. But if one lives with an unmarried woman, he does not render her a zonah.]
English Explanation of Mishnah Sotah
An aylonit [who is incapable of giving birth], an old woman, and one who is unfit to bear children do not drink and do not receive the ketubah. Rabbi Eliezer says: he may marry another wife and be fruitful and multiply with her. The women listed in this section can all, by definition, not have children. An aylonit is a girl who never develops physical signs of maturity. By definition she cannot conceive. The old woman referred to here is one past child-bearing age. The woman who is unfit to bear children refers to one who drank a sterilizing potion. According to the first opinion in the mishnah, it is forbidden for a man to marry these types of women because they cannot bear children, and he is commanded to have children. Since this is a forbidden marriage, she does not drink the water. We should note that in Yevamoth 6:5 we learned that this prohibition only refers to someone who does not yet have children. A man who has children may marry a woman who cannot have children, since he has already performed his commandment. Rabbi Eliezer points out that this marriage too, while originally prohibited, may be maintained if he marries another woman with whom he can have children. Therefore, if she becomes a sotah she can drink the water.