Kiddushin (Halakhic Engagement)
The mitzva of peru u'revu (procreation) has been stated a number of times in the Torah. However, the laws of gittin (divorces) and kiddushin were only mentioned in our parasha. "If a man marries a woman and lives with her… and he wrote her a bill of divorce" (Devarim 24:1). The Torah did not phrase this in absolute terms of requirement or obligation, but simply explained the fact that if a man marries and later wishes to divorce his wife, this is the proper way to proceed.
The Rosh therefore explained that there is no mitzva at all to marry. The only mitzva that is incumbent upon us is peru u'revu. Since people could indeed have children out of wedlock, marriage is not even a necessary prerequisite for this mitzva. It should be noted that the Rosh is obviously alluding to single people whose sexual union would not involve any specific biblical prohibition. The Rosh pointed out that the text of the berakha recited at kiddushin is "who has sanctified us through his mitzvot and commanded us about forbidden relationships." A normal berakha prior to performing a mitzva would state clearly that we are commanded to fulfill a specific mitzva. Since this berakha does not mention any mitzva of kiddushin, but referred only to the prohibition of forbidden relationships, it can certainly not be considered as a berakha prior to a mitzva. It would seem to be categorized as a birkhat ha-shevach (a berakha which is recited as praise to God when we encounter special phenomena). The Rosh discusses various opinions if we need a minyan to recite the birkhat erusin (kiddushin). The kiddushin itself can obviously be performed in the presence of two witnesses, and there is no requirement to have a minyan present. However, there are opinions that the berakha can only be said when there is a minyan. This, too, would indicate that the berakha is not a berakha on the mitzva of kiddushin (which may not even be a mitzva), but is a birkhat ha-shevach which may only be recited if the ceremony takes place within a community, and therefore a minyan is required for the berakha (Rosh, Ketubot I #12).
The Rambam, on the other hand, felt that there is a positive mitzva to have kiddushin, and he counted it as one of the six hundred and thirteen mitzvot (Sefer Ha-mitzvot, Mitzvat Aseh 213). In fact, the Rambam counted it as one of the sixty mitzvot that absolutely must be fulfilled by normal people under normal circumstances. At the end of the mitzvot aseh section of Sefer Ha-mitzvot, the Rambam explains that only sixty of the two hundred forty-eight mitzvot must actually be done by people at all times and in all situations. The mitzva of kiddushin is included in that list.
The Rambam rules (Hilkhot Ishut 3:23) that, whoever performs that actual kiddushin, the groom or his shaliach (agent) must recite the berakha prior to the kiddushin, "just as we make the berakha before performing all the mitzvot." If a person did kiddushin without reciting the berakha, he may not say it after the kiddushin, as that would constitute a berakha le'vatala (a berakha made in vain). What was done is done.
The Rambam obviously disagrees with the Rosh, and maintains that there is mitzva of kiddushin. Moreover, he thinks that the berakha is a regular birkhat ha-mitzva, even though it is phrased in negative terms ("He commanded us about forbidden relationships"), and does not refer to the mitzva of kiddushin directly.
The customary practice that a Rabbi (mesader kiddushin) presides at the kiddushin ceremony and recites the berakha seems to follow the opinion of the Rosh. If the berakha is made over the phenomenon of the wedding ceremony, it could presumably be said by any person. In fact, the obligation of saying this berakha would fall upon the entire congregation. However, according to the Rambam, it seems that the chatan (groom) should actually recite the berakha himself. After all, he is the person who is fulfilling the mitzva now, and therefore should recite the birkhat ha-mitzva himself. The Noda Be'Yehuda (Even Ha'Ezer II 1) cites a responsum of the Rambam which says that if anyone other than the chatan recites the berakha, it is actually a berakha levatala. The custom in Yemen, Krotochin, and other communities was that the chatan himself makes the berakha. I was present at a wedding where Rav Yosef Kapah zt"l, the eminent Maimonidean scholar and adherent, served as mesader kiddushin. I asked him, how could he make the berakha, which according to the Rambam is a berakha levatala. He responded that in Yemen he would not have done so, but in Israel, the Yemenite community did not retain all their customs. He added that there is no responsum of the Rambam which actually states that this would constitute a berakha levatala, despite the testimony of the Noda Be'Yehuda.
In the title description of Hilkhot Ishut, the Rambam defined the mitzva "to marry a woman through ketuva and kiddushin." This phrase implies that men are obligated in the mitzva of kiddushin, and women are not obligated at all. It is obviously impossible to fulfill the mitzva without the consent of a woman, but he must find a wife, and she may remain single if she so desires. This is similar to the mitzva of peru u'revu, which is incumbent only on men and not on women. The Rambam codifies (Hilkhot Isurei Bi'ah 15:2), "A man is commanded to fulfill periya u'reviya, and a woman is not [commanded]." The Rambam wrote similarly (Hilkhot Isurei Bi'ah 21:21) that a man is not permitted to live without a wife. However, a woman has the right to stay single all her life, although the general context of the laws in that chapter refer to proper moral behavior, that than biblical law, nevertheless, it seems clear that the Rambam felt there is no obligation at all for women to marry.
It is therefore noteworthy that the Rambam specifically mentioned that women are exempt from fourteen of the sixty mitzvot which are incumbent upon all people under normal circumstances. He did not mention there that women are exempt from kiddushin. Inasmuch as he was quite meticulous in enumerating fourteen mitzvot from which women are exempt, and he did list those fourteen, it would seem that he thinks that women are obligated in kiddushin, and this would reflect that the Rambam changed his mind about this issue. It should be noted that Ya'akov Levinger, in his seminal article on the sixty mitzvot which are always obligatory, (in HaRambam khe-Philosoph u-khe-Posek), questioned the accuracy of the list as we have it.
In any event, the Rambam in Mishneh Torah and Sefer Ha-mitzvot clearly thinks that there is real obligation for a man to marry. In general, a young man who reaches the age of bar mitzvah is obligated in all mitzvot. However, the well-known mishna (Avot 5:21) says that the age of eighteen is the proper time to marry. The Rambam codified (Hilkhot Ishut 15:2) that a man is obligated from the age of seventeen. He added that if someone reaches twenty years of age and did not marry, he transgressed and negated a mitzvat aseh.
Our chakhamim apparently felt that the lack of maturity and financial independence are sufficient grounds to exempt lads from the mitzva. In fact, the Chida (Birkhei Yosef, Even Ha'ezer 1:7) says that we should not marry today at the age of thirteen. It is obvious that the obligation of learning Torah is a major factor to consider in determining the proper age of marriage. (See Rambam, Hilkhot Ishut 15:2-3; Shulchan Arukh, Even Ha'ezer I.)