We discussed this in our Insights to Kesuvos 48:2, here is the paragraph that is directly pertinent to your question:

"As for the Rambam's proofs (cited above) that Chupah must be Yichud and Ra'uy l'Bi'ah, the Ran and other Rishonim reject them all. The Gemara (2a) that discusses delaying a Chupah because the woman is a Nidah means simply that the groom can insist on making a Chupah later, since he wants a Chupah after which Bi'ah will be immediately permitted. It does not mean that Chupah would not work at all with a woman to whom the groom is prohibited."

I am including the entire Insight referred to above for your benefit, in case you would like to research this matter further.

Best wishes,

-Mordecai

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2) DOES "CHUPAH" REQUIRE "YICHUD HA'RA'UY L'BI'AH?"

OPINIONS: The Gemara discusses a situation of "Nichnesah l'Chupah v'Lo Nivalah," where a woman entered the Chupah but she is still a Besulah. (Apparently, there are witnesses who observed her from the time of Chupah onwards and testify that she is still a Besulah.) We know that Nesu'in may be effected through Bi'ah (see Rashi, DH Ha Stama, and Kidushin 10a), but apparently there is something else called "Chupah" which is a form of Nesu'in that does not involve Bi'ah. What exactly is this "Chupah" and how is it done?

(a) The RAN (Kesuvos 1a of the pages of the Rif) notes the opinion of some Rishonim that Chupah means Yichud, where the man and woman seclude themselves together. The RAMBAM (Hilchos Ishus 10:1), who seems to be of this opinion, takes it even further and writes that the Chasan "has to bring her into his house, seclude himself with her, and separate her for himself." The Rambam (10:2 and 10:6) adds that Chupah serves to acquire her to him only when, at the time of Chupah, the woman is "Ra'uy l'Bi'ah" (fit for the husband to have relations with her), and not if she is Asurah to her husband (such as a woman who is a Nidah at the time of Chupah).

1. The Ran points out that the source for this opinion might be the Gemara in Kesuvos (2a) that discusses a woman who is forced to delay the Chupah because she became a Nidah. Why does the fact that she is a Nidah prevent her from making a Chupah? According to the Rambam, it is clear why Chupah cannot be made with a Nidah; since Chupah comprises Yichud, it is Asur for the Chasan to seclude himself with the Nidah (4a), and, furthermore, she is not "Ra'uy l'Bi'ah," so Chupah will not work even if he does seclude himself with her.

2. A further source for this view of what Chupah is could be the Gemara later (56a), which discusses whether a Chupah serves as a Kinyan if it is not "Ra'uy l'Bi'ah" (such as when the Kalah is a Nidah). The Gemara leaves the question unconcluded ("Teiku"), and the Rambam therefore should be justified in ruling stringently, that the Chupah does not work if it is not Ra'uy l'Bi'ah.

3. Another source might be from the Mishnah in Nidah (44b), which says that only when a girl is three years old does her Nesu'in to a Kohen permit her to eat Terumah. As Rashi in Sanhedrin (55b) explains, when she is younger than three years of age Nesu'in to a Kohen cannot enable her to eat Terumah, because she cannot have a Chupah nor perform Bi'ah to make a complete Nesu'in with the Kohen, since she is not yet "Ra'uy l'Bi'ah."

(b) The RAN, however, proposes a number of strong proofs against this opinion.

1. He proves from our Gemara that Chupah does not of necessity comprise Yichud. The Gemara describes a situation of a woman who entered the Chupah, but we know that she is still a Besulah. If Chupah is Yichud, since the man and woman were secluded together how can we know that she is still a Besulah? The Beraisa earlier (11b), also, describes a woman whose husband performed Nesu'in with her and yet there are witnesses who testify that she did not seclude herself with him or live with him. It seems clear that there is such a thing as Chupah without Yichud.

2. The Ran proves that a Chupah performed with a woman who is not Ra'uy l'Bi'ah is nevertheless a valid Chupah from the Gemara in Yevamos (57b-58a), which describes a Chupah that is performed between an Almanah and a Kohen Gadol, or a Gerushah and a Kohen Hedyot. The Gemara there quotes an opinion (which is the Halachah) that the Chupah takes effect -- even though the woman is not Ra'uy l'Bi'ah since she is prohibited to him!

3. Others prove from our Mishnah both that Chupah does not involve Yichud, and that Chupah may be performed with a woman who is not fit for Bi'ah (such as with a girl less than three years old). The Mishnah describes the effects of "Mesiras ha'Av l'Sheluchei ha'Ba'al" -- the father handing over his daughter to the emissaries of the husband. A number of Amora'im rule that Mesirah accomplishes the same thing as Chupah, and thus the woman may even eat Terumah (if her husband is a Kohen) after the Mesirah.

Mesirah is clearly not Yichud, since it involves no seclusion. If Chupah requires Yichud, then how could Mesirah be able to accomplish a full Nesu'in according to any Amora'im? It must be that Chupah involves simply bringing the woman into the man's home, and that is why some Amora'im hold that Mesirah can accomplish the same thing (since it is the beginning of the transfer of the bride to the groom's home). (BEIS YAKOV, Kesuvos 2a; see also CHUPAS CHASANIM of the Sha'ar ha'Melech, #9.)

Furthermore, how can the Mishnah say that Chupah does not allow a girl less than three years old to eat Terumah, according to the Amora'im who say that Mesirah does allow her to eat Terumah? The Mishnah in Nidah seems to be discussing a situation where the three year old is married, meaning that she is living together in a house alone with her husband. How can that accomplish any less than Mesiras ha'Av (RASHBA, Kidushin 10b)? It must be that the Chupah of the three year old indeed does take effect, even though she is not "Re'uyah l'Bi'ah."

As for the Rambam's proofs (cited above) that Chupah must be Yichud and Ra'uy l'Bi'ah, the Ran and other Rishonim reject them all. The Gemara (2a) that discusses delaying a Chupah because the woman is a Nidah means simply that the groom can insist on making a Chupah later, since he wants a Chupah after which Bi'ah will be immediately permitted. It does not mean that Chupah would not work at all with a woman to whom the groom is prohibited.

The Gemara (56a) which discusses whether Chupah works when it is not Re'uyah l'Bi'ah is not discussing whether Chupah takes effect with regard to Nesu'in when there is no Chibas Bi'ah. Rather, it is clearly discussing whether the woman gets to keep the Tosefes (the additional money) that her husband added to the Kesuvah if the Chupah was not fit for Bi'ah. The Nesu'in, though, does take effect (Ran, 22b of the pages of the Rif, and other Rishonim).

The Mishnah that says that Nesu'in to a Kohen of a girl under three years old does not allow her to eat Terumah does not mean that the Chupah does not make her the Kohen's wife. Rather, it means that in the case of such a young girl, the fear of "Simpon" applies even after her Chupah, since her husband does not fully examine her for blemishes until she reaches the age of three (TOSFOS Yevamos 57b, DH Nisa'as; see also Mishnah la'Melech, Hilchos Ishus 3:11, and Chupas Chasanim #7).

In defense of the view of the Rambam, who says that Chupah is Yichud, it seems that the Rambam does not mean that Chupah requires a total Yichud, where the man and woman seclude themselves behind closed doors. Rather, the Rambam says that the man "must seclude himself with her and separate her for himself " -- the Yichud has to be just enough to show that he is taking her for himself, but it does not necessarily have to be done behind closed doors. If he brings her into his house, even if the doors are open and people can see inside, it is still called a Chupah according to the Rambam. This is indeed the way the BEIS ME'IR (EH 55:1) interprets the Rambam. (See also Insights to 57:1.) This explains how a woman can enter the Chupah, according to the Rambam, and still have witnesses that she was not Niv'alah. This also answers the question from our Gemara, why the father's Mesirah to the emissaries of the husband can accomplish the same as Chupah, even though it is not Yichud. Alternatively, even if Mesiras ha'Av does effect a Kinyan for everything, it still does not accomplish the same as Chupah, as we will discuss later (see Insights to 49:1).

Regarding the Rambam's view that the woman must be Ra'uy l'Bi'ah for the Chupah to be effective, and that Chupah does not work for a Nidah even though it does work for an Almanah marrying a Kohen Gadol, the LECHEM MISHNAH (Hilchos Ishus 10:2) suggests that there is a difference between a woman who is not Ra'uy l'Bi'ah and who will never be Ra'uy l'Bi'ah, and a woman who is presently not Ra'uy l'Bi'ah but who later will be Ra'uy l'Bi'ah. If he marries a woman who will never be Ra'uy l'Bi'ah (such as a Kohen Gadol who marries an Almanah), the Chupah is bringing them together as much as the marriage can bring them together, because he will only be able to live with her b'Isur. But when a man marries a Nidah or a girl less than three years old, although at present she is not Ra'uy l'Bi'ah, the husband intends to be together with his wife later, when she becomes Ra'uy l'Bi'ah. Since right now the woman is not Ra'uy l'Bi'ah, the Chupah is not bringing them together as the marriage eventually will. Therefore it is not a good enough Chupah to marry her to him until she is fully ready for Bi'ah.

How will the Rambam -- and Rashi, who seems to agree to him on this point -- answer the question of the Rashba: Why can a girl under three not eat Terumah after the Chupah? If even Mesirah allows a woman to eat Terumah (although it is not a preparation for Be'ilah)! If there is no cause for concern, after the Mesirah, that the Kalah will share the Terumah with her brothers and sisters (Rashi DH v'Rav Asi), then certainly there is no cause for such concern when the Kalah (who is under three years of age) moves into the house of her Chasan! (We cannot answer that Rav Asi is permitting her to eat Terumah only according to the Mishnah Rishonah, see Mishnah, Kesuvos 57a, since it is quite clear that he is presenting a Halachic ruling, and the Halachah is in accordance with the Mishnah Acharonah.)

Perhaps Rav Asi and those who hold that Mesirah allows the woman to eat Terumah hold that the Nesu'in that does not allow a girl under three to eat Terumah is Nesu'in of Bi'ah (in the father's house, without Mesirah). But if the father hands her over to the husband, then certainly if she enters the husband's house l'Shem Chupah, she is able to eat Terumah, even though she is not Ra'uy l'Bi'ah. (M. Kornfeld)