Resources for Yevamos daf 62

1.      The גמרא  says that according to ר׳ יוחנן if a person had children when he was a נכרי and then converted, he was מקיים פּרו ורבו. The רמב"ם  in הלכות אישות פּרק ט"ו הל׳ ו clarifies that is only if his children convert with him as well. The מגיד משנה there explains that the רמב"ם  understood this from his סברא that it cannot be that you are יוצא the מצוה of פּרו ורבו if the children aren’t Jewish. However, if they do convert, the person has fulfilled his מצוה since the children are always considered to be מיוחס  to him (in regards to this הלכה).

The טורי אבן in ר"ה דף כ"ח ד"ה אילימא asks how the גר could have been יוצא his מצוה of having children by having them when he was a נכרי —after all, we has not a בר חיובא at the time? The מנחת חינוך in מצוה א׳, י"ד says a tremendous חידוש to answer this question: he says that the מצוה of פּרו ורבו is not the מעשה ביאה as that is just a הכשר מצוה. Rather, the מצוה is having the children which he is מקיים every minute the children are alive. The proof to this is that if the children die before the father dies then the father has not fulfilled his מצוה. Consequently, even though the גר had children before he converted, the מצוה continues to occur when he is already a Yid. The מנחת חינוך asks on himself that this is against תוספות in בבא בתרא דף י"ג כופין that says in a case of a person who is חצי עבד וחצי בן חורין that we can’t say עשה דוחה לא תעשה to allow him to marry a בת חורין since he violates the לאו of marrying her when he does העראה and is only מקיים the מצוה of פּרו ורבו at גמר ביאה. Clearly תוספות learns that the מצוה of פּרו ורבו is the מעשה ביאה itself and not having the children as he had suggested in which case the טורי אבן’s question comes back. The קובץ הערות in סימן ס"ט אות כ"ו answers that the מנחת חינוך was right that the קיום המצוה is having the children so the גר can still be מקיים it even though the children were born when he was still a נכרי. However, the מעשה המצוה which is what is necessary to be דוחה the לאו is only by the גמר ביאה. Reb Moshe זצ"ל in אגרות משה,  אבן העזר ב׳ סימן י"ח has a fundamentally different approach. He learns that the מעשה ביאה is the קיום המצוה, whether you have children or not. After all, how can Hashem give a person a מצוה that is not in his control? Rather, anytime the person does a ביאה that is fit to have children, he is מקיים the מצוה of פּרו ורבו. The משנה that says a person needs to have a boy and girl is just saying the שיעור of the מצוה. In other words, once you have a boy and girl the חיוב מצוה stops unless they die in which case it starts again. A משל I would suggest is ביקור חולים. Every time you visit the חולה you do the מצוה. Once the חולה is better, there is no מצוה to keep visiting him. Similarly, each ביאה שראוי להוליד is a מצוה, but once there is a boy and girl there is no longer a need to keep doing the מצוה. Reb Moshe זצ"ל brings a proof from the לשון of the משנה. It doesn’t say “how many children must a person have?” Rather, it says “A person should not stop doing פּרו ורבו until they have a boy and girl”.  Accordingly, if the children were born to the גר when he is a נכרי, perhaps he was not מקיים the מצוה but the fact is the children are here so there is no more מצוה. A possible difficulty on the אגרות משה is that ר׳ יוחנן said the גר was מקיים the מצוה which doesn’t sound like R’ Moshe’s חידוש.