Gemara

(Beraisa): "And they (the elders) speak to him" - they give him proper counsel;

If the Yavam is young and the Yevamah is old or vice-versa, they discourage Yibum. 'Marry someone your own age, and do not invite quarrels into your home.'

61b (Mishnah): A man may not neglect having children unless he has children (and fulfilled Peru u'Rvu).

(Gemara) Version #1: If he has children he may neglect having children, but he should have a wife. This supports Rav Nachman;

(Rav Nachman): Even if a man has many children, he may not remain single - "It is not good that man be alone."

Version #2 - Inference: If he has children, he may neglect having children and having a wife.

Question: This refutes Rav Nachman!

Answer: No. If he lacks children, he must marry a woman who can bear children. If he has children, he may marry a woman who cannot bear children.

One sells a Sefer Torah to marry a childbearing woman only if he lacks children.

62b: Our Mishnah is unlike R. Yehoshua.

(Beraisa - R. Yehoshua): If a man married in his youth, he should marry in his old age. If he had children in his youth, he should have children in his old age - "Sow your seed in the morning, and do not withhold in the evening, for you do not know which will be better."

R. Akiva says, if a man learned Torah in his youth, he should learn in his old age. If he had Talmidim in his youth, he should have Talmidim in his old age - "Sow your seed..."

(Rav Masnah): The Halachah follows R. Yehoshua.

(R. Tanchum): A single man dwells without joy, Berachah or good;

We learn from "You will rejoice, you and your household", "To put Berachah on your household" and "It is not good that man be alone."

In Eretz Yisrael, they say that he lacks Torah and a wall. We learn from "If my helpmate is not with me, Torah is cast from me" and "A female will surround a man."

(Rava bar Ula): He lacks Shalom, for it says "... Shalom is your tent..."

63a (R. Elazar): One without a wife is not called Adam - "He created them male and female and he called their name Adam."

63b (Rav Asi): "Ki Ru'ach mi'Lefanai Ya'atof" - Ben David will not come until all Neshamos in 'Guf' are completed.

65a (Beraisa): If a woman married Reuven and did not have children (in 10 years), to David and had no children, afterwards she may marry only a man who has children.

Rishonim

Rif and Rosh (6:7,9): Even if a man has many children, he may not remain single - "It is not good that man be alone." R. Yehoshua taught that if a man married in his youth, he should marry in his old age. If he had children in his youth, he should have children in his old age - "Seed your seed..." The Halachah follows R. Yehoshua. This is mid'Rabanan. Mid'Oraisa, once he has a male and a female he was Yotzei Peru u'Rvu.

Rosh (9): It seems that the Rif's text did not say 'our Mishnah is unlike R. Yehoshua.' He holds that there is no argument. One should always be married. One must marry a woman who can conceive if he has the money. If he lacks the money, he must sell a Sefer Torah (if necessary) for a fertile woman only if he has not yet fulfilled Peru u'Rvu. According to our texts, this is how Rav Nachman explains the Mishnah, but R. Yehoshua argues. One must sell a Sefer Torah for a fertile woman even if he already fulfilled Peru u'Rvu. R. Yehoshua agrees that fulfilling Peru u'Rvu allows one to keep a wife even if she did not conceive in 10 years. He requires marrying a fertile woman only l'Chatchilah.

Nimukei Yosef (DH Gemara): If one knows that he is infertile he marries an infertile woman, to avoid sin. It seems that anyone fertile should sell a Sefer Torah for a fertile woman. We exclude selling a Sefer Torah only for an infertile woman.

Ba'al ha'Ma'or: The Rif did not say what difference it makes if the Mitzvah to have more children is mid'Oraisa or mid'Rabanan. It seems that in any case one sells a Sefer Torah for children. It seems that the difference applies to one who already fulfilled Peru u'Rvu, remarried after his wife died, and his new wife did not conceive in 10 years. He need not divorce her because the Mitzvah to have more children is only mid'Rabanan. Alternatively, after one's wife died he may not remarry until three Regalim pass. It is permitted only if he did not yet fulfill Peru u'Rvu mid'Oraisa.

Milchamos Hash-m: The Rif merely comes to teach the truth. We must say that R. Yehoshua's law is mid'Rabanan. If not, the argument of Beis Shamai and Beis Hillel was for naught! There are also laws that depend on whether or not it is mid'Oraisa. If it was mid'Oraisa we would lash people until they fulfill it, like we lash one who does not want to fulfill Lulav or Sukah or excommunicate him. Rather, it is mid'Rabanan. It was not an enactment. Rather, Chachamim teach proper conduct. We are not so stringent about it. If an infertile woman became a Sotah, she may drink only if her husband fulfilled Peru u'Rvu mid'Oraisa. The Ba'al ha'Ma'or's distinction is also true. After fulfilling Peru u'Rvu mid'Oraisa he need not divorce one who did not conceive in 10 years, for it is a Safek if she is sterile. The Rif brought Rav Nachman's law to teach the mid'Oraisa law.

Nimukei Yosef (20a DH Masnisin): If one has children, even out of wedlock, he may desist from having more. We do not force due to "Sow your seed..." A Mishnah allows a barren woman to marry a man who fulfilled Peru u'Rvu. It seems that this is even according to R. Yehoshua!

Rambam (Hilchos Ishus 15:7): If a woman was married to Reuven and did not have children (in 10 years), to David and had no children, afterwards she may marry only a man who has children.

Rebuttal (ha'Ramach): The Halachah follows R. Yehoshua, who says that even after fulfilling Peru u'Rvu a man must have more children!

Answer (Magid Mishneh): Here the Rambam gives the mid'Oraisa law. Below (Halachah 16) he brings R. Yehoshua's law, which is mid'Rabanan.

Lechem Mishneh: Perhaps the Gemara said that R. Yehoshua disagrees because it is difficult to say that the Mishnah's law is only mid'Oraisa. Alternatively, the Rambam's text did not say that R. Yehoshua disagrees. However, the Rosh says that if so, one who fulfilled Peru u'Rvu does not sell a Sefer Torah for a fertile woman. The Rambam (Hilchos Sefer Torah 10:2) permits selling a Sefer Torah to get married without this stipulation.

Rambam (16): Even if a man fulfilled Peru u'Rvu, he is commanded mid'Rabanan to continue procreating as long as he is able. Whoever adds a Nefesh to Yisrael is like one who built an entire world. Also, it is a Mitzvah mid'Rabanan that a man not be single lest he come to (lustful) thoughts.

Question (ha'Ramach): The Asmachta for this Mitzvah is "Sow your seed in the morning, and do not withhold in the evening, for you do not know which will be better." This shows that the reason is that we are concerned lest his first children die, and then he did not fulfill the Mitzvah. Why does the Rambam give a different reason?

Answer (ha'Emek She'alah, v'Zos ha'Berachah 165:3): The She'altos says that one who knows that he is fertile must marry a fertile woman to have more children. It because it is a Safek mid'Oraisa; perhaps his children will die. If he was unsure whether or not he is still fertile, he would be exempt for then it is two doubts. The Rambam obligates because 'whoever adds a Nefesh...', not only due to the Safek that his children will die. He learns from the end of the verse "perhaps both (earlier and later children) will be good." This applies even to one who is unsure whether or not he is fertile.

Derech Pikudecha (Mitzvas Aseh 1, brought in Hagahos Tur ha'Shalem 31): It is an Aseh from Divrei Kabalah, which is stronger than a Stam mid'Rabanan. This is why one may sell a Sefer Torah for it!

Poskim

Shulchan Aruch (EH 1:8): Even if a man fulfilled Peru u'Rvu, it is forbidden to remain single. One must marry a fertile woman who can conceive if he has the money even if he has many children. If he lacks the money, he sells a Sefer Torah for a fertile woman only if he has not yet fulfilled Peru u'Rvu. If he already fulfilled it, he marries an infertile woman; he does not remain single. Some say that he sells a Sefer Torah for a fertile woman even if he already fulfilled Peru u'Rvu.

Chelkas Mechokek (10): If he cannot marry any woman without selling a Sefer Torah, he does so to avoid being alone. This is why the Mechaber repeats 'he does not remain single'. Even though the Gemara connotes that one sells to fulfill "Lasheves Yetzarah", avoiding sinful thoughts is more important than Sheves. The Terumas ha'Deshen (cited below) says it is better (for one who has children) to marry an infertile woman if this will avoid strife, but one should not desist from marrying due to strife. The Milchamos Hash-m says so explicitly. The Nimukei Yosef seems to disagree, but one can explain that he agrees.

Rebuttal (Beis Shmuel 15): The Nimukei Yosef disagrees with the Milchamos Hash-m; also the Bach says so. We sell a Sefer Torah only for a Mitzvah, not to avoid an Aveirah. We cannot say that it is proper to be stringent, for this is a leniency regarding selling a Sefer Torah. The Rosh says that one need not sell a Sefer Torah for this, i.e. it is optional, but this is because he permits an individual to sell a Sefer Torah.

Rama mi'Pe'ano (cited in Chelkas Mechokek 11): Whenever the Mechaber brings a second opinion 'some say...', he rules like the first opinion. He brings the latter opinion merely to honor those who hold like it. In some cases the custom is like that opinion.

Beis Shmuel (15): We cannot say that the Mechaber favors the first opinion. The Rosh, Ba'al ha'Ma'or, Ramban, Ritva and Nimukei Yosef explicitly hold like the second opinion. No one explicitly says like the first opinion. The Rosh and Ba'al ha'Ma'or explain that the Rif says so, but the Ramban rejected the proof. The Bach says that Tosfos says so, but this is according to the Mishnah, and the Halachah follows R. Yehoshua.

Rema: If one knows that he is infertile he marries an infertile woman. Similarly, if he has many children and he fears that if he marries a fertile woman it will lead to feuds between his children and his wife, he may marry an infertile woman. This concern does not permit remaining single.

Source (Terumas ha'Deshen 283, cited in Beis Yosef DH uvi'Terumas and Darchei Moshe 6): If one already fulfilled Peru u'Rvu it is better to marry an infertile woman who is not quarrelsome than a fertile, quarrelsome woman. We discourage Yibum if they are far apart in age, lest they quarrel, and do Chalitzah instead (44a). However, even if one cannot find a woman who is not quarrelsome he should not remain alone.

Rebuttal (Chelkas Mechokek 12): Yibum depends on the desire of the Yavam. Some say that Chalitzah is always better! We cannot learn from Yibum to totally exempt from the Mitzvah to have more children.

Defense (Gra 22): Chalitzah does not fulfill the Mitzvah of Yibum (nevertheless we do Chalitzah to avoid strife). This is why Yibum overrides Chayavei Lavin, for one does not fulfill both through Chalitzah (21a).