1. bereishis 1:28 says p’ru ur’vu, be fertile and increase and fill the earth. is that a mitzvah or a bracha, and if it is a mitzvah what exactly does it demand?
2. the mishna in yevamos 61b says a man may not neglect the mitzva of p’ru ur’vu unless he already has children, and then beis shammai and beis hillel argue: beis shammai say two males fulfill it, beis hillel say a male and a female, because the pasuk says male and female he created them. why does the gender mix matter at all for fulfilling the mitzva? and the same gemara adds that even a man with many children may not live without a wife, so is the mitzva about the children or about the marriage?
3. and the mishna in yevamos 65b says a man is commanded in p’ru ur’vu but a woman is not, and rabi yochanan ben beroka argues she is also commanded because the pasuk says and God blessed THEM. why does the mainline halacha put the obligation only on the man? what is the logic of that, and where does that leave the woman halachically, not obligated but obviously central?
bottom line: when has a man actually fulfilled p’ru ur’vu, and how should a person think about the numbers and the genders today?
Thank you Rabbi, thank you very much.
The Sefer HaChinuch gives a very simple explanation. The purpose of פרו ורבו is that the world should be populated, לא תהו בראה לשבת יצרה. It is an especially important mitzvah because without people, none of the other mitzvot could continue to exist.
Halachically, we follow Beit Hillel: a man fulfills the basic mitzvah of פרו ורבו when he has a son and a daughter.
Why exactly Hashem made that the measurement, and why the formal obligation is placed on the man rather than the woman, I do not think we need to invent explanations for. The Gemara derives the rules, the Poskim codify them, and that is the Halachah. There are mefarshim who discuss possible reasons, but ultimately some questions about why Hashem structured men and women differently may have to wait until we can ask Him.
That also does not mean that once a man has a boy and a girl, marriage or additional children suddenly lose their value. Chazal separately teach that even after fulfilling פרו ורבו, a person should not simply live without a spouse. Marriage is much more than a mechanism for producing the minimum number of children.
And I would not approach having children as, “I need another one to check off a mitzvah.” Children are an enormous responsibility. If you are able to raise them properly, support them, love them, educate them, and you want to continue building your family, God bless you.
So the practical answer is simple: the basic Torah mitzvah is fulfilled with a son and a daughter. Beyond that, do the best you can, build the family you can responsibly handle, and do not turn the numbers into an obsession.