1) WHEN A KETANAH DOES NOT NEED "MI'UN"
QUESTION: The Gemara concludes that according to Rebbi Yehudah ben Beseirah (whose opinion is the Halachah), if a Ketanah is married with Kidushin d'Rabanan and then she accepts Kidushin from another man, her acceptance of Kidushin serves as Mi'un and the first Kidushin is annulled (even if she was married with Nisu'in to the first husband). The Gemara questions whether any Tana argues with this ruling. The Gemara cites a Beraisa which quotes the Rabanan who do not agree entirely with Rebbi Yehudah ben Beseirah's ruling.
Why does the Gemara cite a Beraisa to prove that a Tana disagrees with Rebbi Yehudah ben Beseirah, when such a Tana is quoted in the Mishnah itself? The Mishnah (107b) states that if a relative marries off the Ketanah without her knowledge or consent ("she'Lo mi'Da'atah"), the Kidushin does not take effect and she does not need to do Mi'un.
If the Mishnah agrees with the ruling of Rebbi Yehudah ben Beseirah, its ruling is unnecessary. If the Ketanah's acceptance of Kidushin from another man constitutes Mi'un and, consequently, the annulment of her earlier Kidushin, it makes no difference whether the first Kidushin was done with her consent or not. As soon as she accepts Kidushin from another man she is considered to have done Mi'un with the first, and the first Kidushin is annulled. Whether she was married off with her consent or without her consent, if she wants to marry someone else she may do so and does not need to do Mi'un (either because the first Kidushin did not take effect against her will, or because her acceptance of Kidushin from the second man nullifies the first Kidushin). It must be that the Mishnah disagrees with Rebbi Yehudah ben Beseirah and maintains that her acceptance of Kidushin from another man does not constitute Mi'un, and therefore it makes a difference whether her first Kidushin was done with her consent (and took effect) or without her consent (and did not take effect). Why does the Gemara not cite the Tana of the Mishnah as the Tana who disagrees with Rebbi Yehudah ben Beseirah?
(a) The RAMBAM (Perush ha'Mishnayos) explains the Mishnah in its most straightforward sense. Since the Ketanah's Kidushin d'Rabanan ("Kidushei Mi'un") is not valid in that case, she is allowed to marry someone else l'Chatchilah without Mi'un.
The Mishnah may agree with the opinion of Rebbi Yehudah ben Beseirah, and the difference between whether the Kidushin takes effect against her will (but her acceptance of Kidushin from someone else nullifies it) or whether the Kidushin does not take effect against her will (and thus she is free to marry someone else) is as follows. The YAM SHEL SHLOMO (13:9) explains that the Rambam maintains that Rebbi Yehudah ben Beseirah's ruling applies only b'Di'eved: if the Ketanah accepted Kidushin from someone else, her acceptance annuls her previous marriage. L'Chatchilah, however, she must first do Mi'un and only afterwards may she accept Kidushin from someone else. In the case of the Mishnah, in which the Ketanah was married off without her consent, she may marry someone else even l'Chatchilah without Mi'un. (See MAHARSHA.)
(b) The Rashba cites RABEINU CHANANEL who explains that the Mishnah means that if she was married off without her consent, when she reaches adulthood and has not had relations as an adult with her husband she is not considered married to him and she is allowed to leave him without a Get. In a case in which Kidushin does take effect when she is a Ketanah (such as when she consents), when she becomes an adult she may leave her husband only with a Get, even if she did not have relations with him when she was an adult. Since the Mishnah's ruling (Kidushin accepted by a relative on behalf of a Ketanah against the Ketanah's consent does not take effect) affects the Ketanah's status only when she reaches adulthood, the Mishnah may follow the opinion of Rebbi Yehudah ben Beseirah.
However, this issue apparently depends on a Machlokes Amora'im later (109b). The Amora'im disagree whether or not Kidushei Mi'un automatically becomes Kidushin d'Oraisa when the girl becomes an adult. Consequently, according to the opinion that it remains a Kidushin d'Rabanan, there still should be no difference between being married off with her consent and being married off without her consent; in both cases, she should not need Mi'un because her acceptance of Kidushin from the second man annuls the first Kidushin.
(c) The RASHBA disagrees with Rabeinu Chananel and asserts that even Kidushei Mi'un done without her consent also requires Mi'un if she wants to marry someone else. She has the status which Rebbi Eliezer (in the end of the Mishnah) gives to every Ketanah who is married with Kidushei Mi'un. Even though Rebbi Eliezer calls her a "Mefutah," he agrees that she needs Mi'un if she wants to leave her husband.
Accordingly, the Mishnah may agree with the view of Rebbi Yehudah ben Beseirah. The Mishnah agrees that either Mi'un or acceptance of Kidushin from a second man nullifies the first Kidushin. When the Mishnah says that Kidushin done without her consent does not need Mi'un, it means only that she is considered a Penuyah (not married) as far as the husband is concerned (for example, he does not inherit her, he has no claim to the objects she finds, he may not become Tamei for her or give her Terumah d'Rabanan to eat if he is a Kohen). If she wants to leave her husband and marry someone else, she does need Mi'un or acceptance of Kidushin from the second man.