Sotah, Chapter 4, Mishnah 3. In Mishnah 1 of this chapter we saw that a woman who is forbidden to her husband not because of the laws of sotah, but because she is a forbidden relative prohibited by the Torah, cannot undergo the sotah process at all, and she does not drink the sotah waters. It is a condition that she be a woman who is fit for her husband and suited to him. Our Mishnah opens with the question: what is the law when the prohibition is not from the Torah, but a rabbinic obligation?
A woman pregnant by another man and a woman nursing another man's child:
The Mishnah deals with a woman who was married to another man, and her husband died or divorced her, and she is pregnant by him or nursing his infant. By rabbinic law she may not marry until she gives birth or until she finishes nursing, because if she becomes pregnant it could endanger the life of the other child.
"Me'uberes chaveiro u'meineke chaveiro lo shosos v'lo notlos kesubah, divrei Rabbi Meir" - according to the Tanna Kamma, whose view is that of Rabbi Meir, she does not drink, because it is rabbinically forbidden for him to be married to her, and she also does not receive her kesubah. Rabbi Meir's reasoning: one who marries a woman pregnant by another man or nursing another man's child is required to divorce her, and he may not take her back later.
"VaChachamim omrim: yachol hu l'hafrishah u'l'hachazirah l'achar zman" - the Sages disagree and hold that this is not a permanent separation: he separates her from himself and takes her back later. Since it is in his power to marry her again, she is fit for him and suited to him, and the impediment applies only to this present time. Therefore, if he warned her and she secluded herself, the law of sotah applies to her.
An aylonis, an elderly woman, and one who is unable to bear children:
The Mishnah raises a further case: "Aylonis u'zekeinah v'she'einah re'uyah leiled lo shosos v'lo notlos kesubah". An aylonis is a barren woman who cannot bear children, because she lacks the physical features essential for childbearing; an elderly woman - who has passed the age of fertility; and one who is unable to bear children - who cannot give birth for some other reason, such as that she drank a potion that rendered her barren. According to the Tanna Kamma she too does not drink, since her husband cannot fulfill the mitzvah of being fruitful and multiplying with her.
"Rabbi Eliezer omer: yachol hu lisa ishah acheres v'lifros v'lirbos heimenah" - Rabbi Eliezer holds that she is fit to be married to him, for if he has no children it is in his power to marry another woman and have children with her, since by Torah law he may marry more than one wife. The obligation to be fruitful and multiply does indeed rest upon him, but that is not enough to make her a woman to whom he cannot be married, merely because she cannot bear children.
All other women:
"U'she'ar kol hanashim o shosos o lo notlos kesubah" - a rule we have already learned: all other women either drink, or, if they do not wish to drink, they cannot collect their kesubah. A woman who says "I will not drink" - she leaves without a kesubah.
In summary: in this Mishnah we examined a woman who is forbidden to her husband because of a rabbinic obligation - a woman pregnant by another man and a woman nursing another man's child: according to Rabbi Meir she does not drink and does not receive a kesubah, while according to the Sages, since it is in his power to separate from her and take her back later, she is fit for him and the law of sotah applies to her. So too regarding an aylonis, an elderly woman, and one who is unable to bear children: according to the Tanna Kamma they do not drink and do not receive a kesubah, while according to Rabbi Eliezer they are fit for him, since he can marry another woman and have children with her. And all other women - either they drink, or they do not receive a kesubah.