1) KIDUSHIN WITH A NIDAH
QUESTION: The Gemara asks that if the source that Kidushin with an Ervah does not take effect is the Hekesh which compares all Arayos (Vayikra 18:29) to the Ervah of "Achos Ishah" (Vayikra 18:18), Kidushin performed with a Nidah also should not take effect. Moreover, a child born through a union with a Nidah should be a Mamzer, since the prohibition against relations with a Nidah (an Isur Kares) is also mentioned in the Parshah of Arayos. Everyone agrees, however, that the Kidushin takes effect when one marries a woman who is a Nidah, and that a child born from such a union is not a Mamzer. The Gemara answers that a verse (Vayikra 15:24) explicitly teaches that Kidushin takes effect with a Nidah.
The RAMBAM (Hilchos Ishus 4:12) writes that "one who is Mekadesh any of the Arayos has done nothing, for Kidushin with an Ervah cannot take effect, except for Kidushin with a Nidah; when one is Mekadesh a Nidah, she is Mekudeshes with complete Kidushin, but it is not appropriate to do so."
The commentaries on the Rambam point out that there is no clear source in the Gemara for the Rambam's ruling that "it is not appropriate" to perform Kidushin with a woman while she is a Nidah. What is the source for the Rambam's ruling?
(a) The MINCHAS CHINUCH (Hashmatos to #206) suggests that the Gemara here is the source for the Rambam's ruling. The Gemara states that marrying an Ervah involves two prohibitions: the first is an Isur "l'Chatchilah," and the second is an Isur "b'Di'eved." The Gemara compares Nidah to the other Arayos, and it needs a special verse to prove that Kidushin with a Nidah takes effect. The verse which it cites, however, proves only that b'Di'eved the Kidushin takes effect; it does not prove that one may be Mekadesh a woman l'Chatchilah while she is a Nidah. When the Rambam writes that "it is not appropriate" to be Mekadesh a Nidah, his source is the Isur l'Chatchilah implied by the Gemara here.
The Minchas Chinuch asks, however, why the Rambam chooses to express this Isur specifically with regard to one who is Mekadesh a Nidah and not with regard to Kidushin with any other Ervah.
According to the logic mentioned earlier (see Insights to Kidushin 67:3), the reason why the Rambam mentions this Isur only with regard to a Nidah is clear. As mentioned earlier, if an act of Kidushin has no validity, it is not possible for that act to be forbidden by the Torah. Only if the act takes effect can the act of Kidushin be forbidden l'Chatchilah. Accordingly, Kidushin with a Nidah differs from Kidushin with other Arayos. Since the Kidushin takes effect, there can exist an Isur l'Chatchilah to be Mekadesh her, as the Gemara here implies. In the case of other Arayos, the Kidushin does not take effect and thus there can be no prohibition against performing an act of "Kidushin" with them. The Rambam therefore rules that there is a prohibition against performing Kidushin only in the case of marrying a Nidah, where the Kidushin actually takes effect.
(b) Other Acharonim note that the phrase the Rambam uses -- "it is not appropriate" -- with regard to an act of Kidushin with a Nidah connotes an Isur d'Rabanan and not an Isur d'Oraisa. If the Rambam maintains that a Lo Ta'aseh prohibits one from performing Kidushin with a Nidah, he should write explicitly that such an act is "Asur" and not merely that "it is not appropriate."
The MAGID MISHNEH cites an explanation which suggests that the Rambam is concerned that when a man marries a woman while she is a Nidah perhaps they will transgress the prohibition of bodily contact, and thus the Rambam advises against performing such a marriage (as a "Seyag l'Isur Nidah d'Oraisa").
2) A NOCHRI'S KIDUSHIN WITH AN "ERVAH"
OPINIONS: The Mishnah (66b) states that when one attempts to perform Kidushin with a woman who is forbidden to him as an Ervah, the Kidushin does not take effect at all. Does this Halachah apply to a Nochri as well? One of the seven Mitzvos of Bnei Noach is the prohibition against living with one's close relatives. When a Nochri performs Kidushin (through Bi'ah, the only method of Kidushin for a Nochri) with his sister, does the Kidushin take effect? If the Kidushin takes effect, another Nochri who has relations with her is punishable with death for adultery with an Eshes Ish. If the Kidushin does not take effect, another Nochri may marry her.
(a) The MINCHAS CHINUCH (191:2) quotes the MISHNEH L'MELECH (in Parashas Derachim) who discusses this question (Hilchos Melachim 9:10). He apparently assumes that a Ben Noach has the same laws as a Jew in this regard, and the Kidushin does not take effect.
(b) The Minchas Chinuch disagrees based on the Gemara here, which discusses at length the source for the rule that Kidushin does not take effect with an Ervah. The verses which the Gemara cites refer exclusively to Jews. Since no explicit verse states otherwise, one must assume that a Nochri's Kidushin with an Ervah does take effect.
3) THE "CHUMRA" OF AN ACT OF KIDUSHIN THAT DOES NOT TAKE EFFECT
QUESTION: The Gemara asks why the Hekesh compares all Arayos to "Achos Ishah" to teach that the Kidushin does not take effect (and that a child born from a union with an Ervah is a Mamzer), when instead it could compare all Arayos to Nidah to teach that the Kidushin does take effect (and that the child is not a Mamzer). The Gemara answers that when a Hekesh can teach one of two laws, a lenient law ("Kula") or a stringent law ("Chumra"), the Hekesh teaches the stringent law.
The Gemara clearly understands that an act of Kidushin which does not take effect is a Chumra, and Kidushin which takes effect is a Kula. The opposite, however, should be true. When the Kidushin takes effect, the woman has the status of an Eshes Ish and is forbidden to the rest of the world (a Chumra), but when the Kidushin does not take effect she is permitted to marry anyone she wants (a Kula). Why is it a Chumra when the Kidushin does not take effect? (RISHONIM)
(a) TOSFOS and other Rishonim answer that the Chumra to which the Gemara refers does not involve the actual Kidushin itself. Rather, the Chumra refers to the consequences which the Kidushin (or lack thereof) has on other Halachos. Since a child born from a union in which Kidushin does not take effect is a Mamzer, the Halachah that the Kidushin does not take effect is considered a Chumra.
(b) The RITVA explains that the Gemara indeed refers to the Kidushin itself (that is, whether or not it takes effect) when it refers to "Chumra" and "Kula." If the Kidushin does not take effect, it must be the Isur Arayos (the prohibition against living with an Ervah) that prevents the Kidushin from taking effect. The severity of an Isur may be measured by assessing whether the Kidushin takes effect when one transgresses that Isur. If the Kidushin does not take effect, it is a sign that the Isur is very severe.
Although it is true that when Kidushin takes effect a new Isur of Eshes Ish is created (which is a Chumra), when one judges the gravity of the act itself he must gauge its severity by the act's inability to take effect.