A man who has already fulfilled the mitzva of procreation, even one who has been privileged to have many children, if his wife dies, there is a mitzva for him to remarry a woman who can still bear children, in order to continue fulfilling the mitzva of procreation (below, 5:6). However, if it would be difficult for him to raise more children – whether because of his age, or because supporting them would be too hard, or because he is worried about acrimony between his children from his first wife and his second wife and her children – he may refrain from marrying a woman who can still bear children (see below, 5:6). However, it is still a mitzva for him to remarry a woman who cannot have children, as a person is in a state of wholeness only when married. As the Sages said, “Any man without a wife is not a man” (Yevamot 63a); and “Any man without a wife lives without joy, without blessing, without goodness, without Torah, without fortification, and without peace” (ibid. 62b). This is the Sages’ instruction: “Even if a man already has a number of children, he may not remain without a wife, as it says (Bereishit 2:18), ‘It is not good for man to be alone’” (ibid. 61b). He will thus have the privilege of continued fulfillment of the mitzva of ona and will also prevent himself from having sinful thoughts.
Rambam likewise rules: “There is a rabbinic mitzva for a man not to remain without a wife, so that he does not come to have sinful thoughts” (MT, Laws of Marriage 15:16). This mitzva is so important that some say that the Sages’ permission to sell a Torah scroll to enable a couple to get married applies even when the bride is too old to conceive. This illustrates the importance of getting married, fulfilling the mitzva of ona, and avoiding improper thoughts (below, 5:21).
If a man has aged to the point that he no longer desires a woman and no longer yearns to fulfill the mitzva of ona, he is not obligated to remarry, if he has already fulfilled the mitzva of procreation. Nevertheless, even someone whose desire has dissipated fulfills a mitzva if he marries and lives happily and lovingly with his wife – both because being married is the proper human condition, through which one fulfills most completely the interpersonal mitzvot, and because he fulfills the mitzva of ona. However, if he is worried that remarriage may be painful for him, because he might not have a loving relationship with his second wife, he is not obligated to marry, as long as there is no concern that he will have sinful thoughts. Indeed, many great rabbis did not remarry after being widowed (Ramban and Me’iri on Yevamot 62b; Ḥokhmat Adam 123:6; AHS 1:7).