Resources for Yevamos daf 64
1. The משנה says that if one has been married to a woman for ten years and didn’t have children with her, he cannot continue to be מבטל from פּרו ורבו. רש"י says that he must either divorce her or marry an additional wife. רש"י in כתובות דף ע"ז ע"א ד"ה ליתני sounds like מדרבנן he must divorce her since we are afraid that if he doesn’t he won’t end up marrying an additional wife. However, the ריטב"א here is explicit that if both the man and first wife agree, then he can take an additional wife and doesn’t need to divorce the first. Moreover, the ריטב"א says the tremendous חידוש that even where חרם דרבינו גרשום exists, one can take a second wife in this case since he never said his חרם in cases like this. In fact, in שו"ת רבינו גרשום סימן מ"ב it says explicitly that רבינו גרשום said he did not intend his חרם for the case of a couple who didn’t have children. However, the ערוך השלחן in סימן א׳ ס"ק כ"ה says that the גדולי אחרונים would not allow a person to take a second wife or be מגרש his first wife בעל כרחה without a היתר מאה רבנים.
2. There is a discussion in the ראשונים about which cases were included in the "לא ילדה עשר שנים" of our משנה. The ריטב"א and רשב"א both bring a ירושלמי that says that if all the person’s children died and his wife didn’t have more children ten years after that, he must marry another wife. The ירושלמי must understand that this is included in "לא ילדה". What if they had only one child instead of the boy and girl required to be מקיים the מצוה? What if they had only all boys or all girls? The רשב"א says that if they at least have one child, the simple reading of our משנה is that they do not need to get divorced because she was ילדה. Nonetheless, our משנה may be לאו דוקה and לא ילדה could mean they did not have a sufficient number of children to be מקיים the מצוה. The רשב"א comes out לקולא that any child is enough to not need to get remarried. The ריטב"א adds that even according to the other interpretation of the משנה, it is פּשוט that if she has all boys or all girls they don’t need to get divorced because who said it will be any different with another lady? However, the ריב"ש in סימן ט"ו holds that even if he many boys or many girls we would not allow him to marry a woman who was too old to have children since he wasn’t yet מקיים the מצוה.
Many אחרונים contend with a fundamental question here: we know a person isn’t obligated to give up more that a fifth of their money to do a מצוה. So why is פּרו ורבו different? Many happy couples would gladly give a fifth of their money to stay married. Moreover, the פּתחי תשובה in אבן העזר סימן קנ"ד אות כ"ה quotes the תשובות בגדי כהונה who adds that this case is really worse than other מצוות since it’s only a ספק whether marrying another lady will help as the issue in having children may be from him! He actually says that this is the reason there is a מחלוקת in כתובות whether בית דין forces a person to marry another wife in this case whereas everyone agrees that we force people in general to do מצוות. The אבני נזר in אבן העזר סימן א ס"ק ח says that the reason פּרו ורבו is different is in order שלא ימחה שמו מישראל. We know the תורה even allowed the איסור כרת of אשת אח for this reason, so we see that carrying on the person’s family legacy is considered crucial and warrants the חיוב לישא אחרת. However, once there is one child, this has been accomplished so they do not need to get divorced (this was to explain those who hold that one child is enough to not get divorced.) It should be noted that many recent great and famous Rabbonim unfortunately did not have children and did not marry anyone else (the חזון אי"ש, Rav Tzvi Yehuda Kook, the Lubavitcher Rebbe, Reb Henoch Lebowitz, זצ"ל, and others.)
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