The poskim disagree about whether a man fulfills the mitzva of procreation by having a son and daughter through an adulterous sexual union that renders them mamzerim. Poskim also disagree about whether a mamzer has a mitzva to marry a woman whom he is permitted to marry – a mamzeret or convert – and have a son and daughter, even though these children will also be mamzerim.
Those who maintain that the mitzva applies to them adduce proof from the Talmud’s statement that the laws of sota apply to mamzerim. That is, if there is reason to suspect that the wife has committed adultery, we erase God’s name and give her the infused water to drink in order to make peace between the husband and wife, even though any children they go on to have will be mamzerim (Sota 26a). The implication is that mamzerim do fulfill a mitzva when they have children.
Others say that it is preferable for a mamzer not to have children, so as to avoid increasing the number of mamzerim. Proof for this view lies in the suggestion of the Sages that a mamzer marry a slave woman in order to purify his offspring. In order to avoid passing on his mamzer status, he may marry a woman from a gentile nation who has been acquired by a Jew. After the birth of their children, he can arrange for their freedom. Their status will be that of freed slaves – Jewish and not mamzerim. This course of action is recommended even though he will not fulfill the mitzva of procreation, because the children born of this union are not considered of his halakhic lineage. We see that it is preferable for a mamzer not to fulfill the mitzva of procreation, so that he will not pass on the status of mamzer to his offspring.
Another reason that supports those who maintain that having children who are mamzerim is nevertheless a fulfillment of the mitzva can be found in the Talmud, which concludes that the halakha follows R. Yose that in the future, all mamzerim will be purified and permitted to marry any Jew (Kiddushin 72b). Some say this purification will be limited to those mamzerim who are not known as such (MT, Laws of Kings 12:3; Ran; Tosafot Ha-Rosh). Others say that even known mamzerim will be declared pure at that time (Ramban and Rashba).
. Those who maintain that a mamzer is obligated to fulfill the mitzva of procreation include R. Yaakov Emden (She’elat Ya’avetz 2:97) and R. Ben-Zion Meir Ḥai Uziel. However, it seems that even they would agree that it is preferable for him to have children with a female slave, thereby purifying the children of this taint. Even though he would not fulfill the obligation of procreation because the children are not halakhically considered of his lineage, he will nevertheless fulfill the mitzva of populating the earth (shevet), which is a more sweeping and important mitzva.
Others maintain that even though a mamzer does fulfill the mitzva of procreation by having children who are mamzerim, he should not do so le-khatḥila. Therefore, if he does not have the option of marrying a female slave (Kiddushin 69a), it is preferable for him not to fulfill the mitzva of procreation. (He may marry a convert or a female mamzer who cannot conceive, or they can use birth control.). This is the position of Netivot La-shevet, EH 1:8 and Minḥat Ḥinukh 1:22. It would also seem to accord with y. Yevamot 8:2, which records that, according to R. Yehuda, two mamzerim may not marry each other, so as not to increase the number of mamzerim (Responsa Pnei Moshe, EH 1:1).