The mitzva of p'ru ur'vu, of producing children, plays a prominent role in the first two parshiyos of the Torah, B'raishis and No'ach. Adam and Chava are commanded "p'ru ur'vu u'mil'u es ha'aretz v'kivshuha" - "be fruitful and multiply, fill the land and conquer it" (1:28). No'ach and his family are similarly commanded, "p'ru ur'vu umil'u es ha'aretz" - "be fruitful and multiply, and fill the earth" (9:1). Chazal elaborate on the centrality of this fundamental commandment, which causes us to partner with Hashem, so to speak, in the acts of creation (see Y'vamos 63b). Halacha goes to great lengths to encourage and foster its fulfillment (see Megilla 27a).
Curious then is the odd exception of women from this mitzva. As Rambam records, based on the majority view in the mishna in Y'vamos (65b), "ha'ish m'tzuveh 'al p'ru u'rvu v'lo ha'isha" - "a man is obligated in the mitzvah of producing children, but not a woman." A double question immediately surfaces. First, the Torah addresses both man and woman in the commandment to Adam and Chava and seemingly so as well with respect to Noach and his children and their wives. It is this first question which leads R. Yochanan b. B'rokah in the mishna (ibid.) to dispute the majority ruling that women are exempt and claim that women are indeed equally obligated. "'Al sh'neihem Hu 'omeir ... p'ru u'rvu" - the plural of "p'ru ur'vu" indicates that Hashem is addressing both Man and Woman. Second, conceptually, it is difficult that those who actually carry and have the children are exempt![1]
In answer to the first question, the Gemara in Y'vamos (ibid.) writes in the name of R. Elazar b. R. Shimon that although the commandment to Adam and Chava is written in the plural, this is followed by the word "v'kivshua" - "and conquer it", which, although read in the plural, is written in the singular, VKVSH, without a vav before the final hei. Since it is the norm of the man to conquer and not of the woman, the Torah indicates that only men are obligated in the mitzva preceding this word. Rav Yosef proves the exemption from the fact that Ya'akov Avinu is commanded: "p'rei ur'vei" (VaYishlach 35:11), written in the singular. This presentation still leaves the following questions unanswered. Why did the Torah write the commandment to Adam and Chava in the plural, only to modify its meaning with the following word VKVSH written in the singular? Furthermore, the commandment is written in the plural to No'ach and his sons and presumably their wives without a subsequent modification. Finally, why is the commandment written in the singular only to Ya'akov Avinu?
R. Meir Simcha of Dvinsk zt"l, in his Meshech Chachma commentary, presents a brilliant approach both to the technical and conceptual questions raised above. His main theme revolves around the concept that "d'racheha darchei no'am", the ways of Torah are pleasant and do not demand of mankind more than they can handle and do not command the person to fundamentally go against the nature implanted within them. It is for this reason that the Torah commands fasting only one day a year and even then provides a mitzva to eat the day before. It similarly permits y'fas toar in a situation that would be extremely difficult to resist. Similarly, since pregnancy and childbirth introduce a risk factor to the woman's health and even life, the Torah does not obligate her in this mitzva. Instead, the Torah relies on the inherent nature and desire of women to have and nurture children. In this way, a woman "answers the call" to have children without being "forced" to endanger herself. She does so of her own volition in a pleasant and self-fulfilling manner.
For this reason, according to Rav Yosef - who quoted the proof from Ya'akov Avinu - Adam and Chava indeed were both obligated in the mitzva since at the time of the commandment the pain and risk of pregnancy and childbirth were non-existent.[2] These were only a function of the new reality caused by the sin of partaking of the fruit of the Eitz Hada'as (B'raishis 3:16). Only after this sin, was the mitzva limited to men only. A careful reading of the passage quoted above to No'ach also indicates this. There the Torah states (9:1): "and G-d blessed No'ach and his sons and said to them p'ru ur'vu." In this verse, their wives are omitted, even though they are mentioned previously concerning the entrance and exit into the ark, since they were no longer commanded in the mitzva. The usage of the plural is because No'ach and his sons are being addressed. Ya'akov Avinu, being addressed alone, without his sons, is commanded in the singular, since his wives were exempt from the mitzva.
This approach of R. Meir Simcha appears to be parallel to a major theme presented often by Rav Yosef Dov Soloveitchik zt"l.[3] G-d has prescribed two law systems: the moral law, the mitzvos of the Torah, and the natural law, the Laws of Nature. As legislator of the natural law, G-d is referred to as Elokim, "B'reishis bara Elokim". As legislator of the moral law, G-d is referred to as by the tetragrammaton, YKVK. When Adam is commanded (2:16) not to eat of the Eitz HaDa'as, the Torah refers to G-d as "YKVK Elokim" (2:16). When G-d reveals Himself to the Jewish people at Har Sinai, he begins "I am YKVK elokecha, your G-d, who took you out of Egypt." Both law systems command us and obligate us; both have consequences. Fulfilling or violating the natural law leads to obvious consequences. A person who eats healthfully and exercises regularly will be of good health. He has "obeyed" the natural law. One who does the opposite will usually suffer from poor health. A person who jumps off a roof, "violating" the rule of gravity, will perish. The moral laws, the mitzvos of the Torah, although their consequences are not as apparent and not as immediate, are equally fundamental. Chazal even teach us that it was the Torah that preceded the world and served as its blueprint and thus are much more fundamental. It is for this reason that even the 'Aseres HaDibros, the description of the moral law, are preceded by "vay'dabeir Elokim es kol had'varim ha'eileh laimor" (Yisro 20:1). The usage of the name "Elokim", the name associated with Hashem as Creator and "legislator" of the natural law, highlights the link between the natural law and the moral law. Just as the natural law is inherent within creation and carries with it fundamental consequences, so too, and even more so, with the moral law.
The insights of these two Torah giants highlight a central theme of our Torah. That which G-d commanded us it not against our nature. It is the entire purpose of our being and existence. All of our talents and characteristics which we are granted by our Creator are given to us to channel and utilize in his service.
[1] Ran, (Kiddushin 41a), indicates that women clearly "assist" in this mitzvah, a status which has halachic ramifications, but it is still conceptually difficult why they are not directly obligated.
[2] Perhaps this approach can explain even R. Elazar b. R. Shimon's view. Although the Torah alludes to the fact that eventually women would be exempted as indicated by the singular VKVSH, Adam and Chava themselves were both equally commanded.
[3] I heard this on a taped lecture on the Aseres HaDib'ros. The theme is mentioned in Lonely Man of Faith and elsewhere.