One young man had one testicle removed surgically due to illness. The Achronim consider on in this situation "mutar lavo bekahal" (a krus shafcha and petzua dakah - eunochs and similar - may not marry into the khal. They can only marry people also forbidden to marry into the khal, like mamzeirim, and they are also allowed to marry geirim.) based on the opinion of Rabbeinu Tam and Sefer haTrumah that if a man has one testicle he is mutar lavo bekahal, combined with Rambam's opinion that when one loses [a testicle] from illness, that is considered an act of G-d, and not a man-made act, see Shulchan Aruch (E"H 5, Pischei Teshuva 7 the responsa of Rav Chaim of Volozhin). However, this decision is not accepted by all opinions, but in a pressing case, like this situation, the poskim are lenient, and he received a psak that he may marry into the khal.
He asked if he must disclose this fact to the matchmakers. This question has two parts: 1) if he does not disclose it, it may be a faulty kiddushin, and 2) even if it is not a faulty kiddushin, it may be onaah, and misleading others who would think he has no defect in that realm.
Regarding the first question, Pischei Teshuva (YD 31:2) brings the Chasam Sofer's opinion that when an animal has a blemish which we would permit only in a case of a great monetary loss had we been strict about that blemish, that heter only applies to the owner of the animal. But he may not sell it... Chasam Sofer says he is close to certain that the owner must disclose the defect to a prospective buyer, and if he did not, the sale is invalid. Similarly in our case, the defect must be disclosed.
However, we can explain that our case will not be a mekach taos because Rav Huna holds (Kesuvos 101b) if the woman was prohibited to the man only on the level of a regular prohibition (not kareis or misas beis din) she receives her kesubah. From his ruling Rav Huna held the marriage certainly took effect. For it is only effective because of doubt, she would not receive her kesubah (see Kesuvos 73b). Even according to the Halacha that accepts Rav Yehuda's opinion that if he was not aware of the lav, she does not receive her basic kesubah, Noda Biyehuda (Tinyana 162) concludes (in the hashmatos) that the kiddushin takes place completely, and this is also the conclusion of Achiezer (E"H 10) that even where the marriage was a violation of chayavei lavin, it is not kiddushei taos. Therefore, our case which is only a doubtful concern of violating a lav - even to the Tiferes Tzvi and Bris Yakov that Achiezer brings who hold if they were married with a lav that they had not realized, their marriage only took effect midirabanan (meaning midioraissa the marriage was not effected and a divorce is required Rabbinically - Ed.), and I also found this is the opinion of Chazon Ish (79:17) that a get is only required in a case of marriage violating a lav that was not realized is only a get midirabanan or misafeik, since she does not have a kesubah as the Gemara (Kesuvos, perek Hamadir) compares get to kesubah (where there is kesubah, a get is required, and vice versa). All of that is if there was a violation of a negative commandment (lav). But violating a positive commandment is explained by Rambam (Ishus 24:4) the woman does receive her kesubah even though they did not realize they were not acting in accordance with the mitzvah, the kiddushin takes effect definitely because it is not as severe [as violating a negative commandment -Ed.] [and see Or Samei'ach there who brings a strong proof to Rambam's opinion], and this is the Halacha as codified by Shulchan Aruch (EH 116, and see Biur HaGra there), since a non-realized positive mitzvah. So our case, since most poskim allowed him to marry into the kahal, there is no doubt of kiddushei taos.
This does not appear to contradict the ruling of Chasam Sofer that if a merchant did not inform the buyer of the possible defect it may be mekach taos - we cannot compare merchandise to marriage. For the sale of a defective - non-kosher - animal is a mekach taos (see Bechoros 37), and even selling something prohibited Rabbinically is a mekach taos, as Rambam and Shulchan Aruch (C"M 234) rule and a marriage in violation of a lav according to Rav Huna is not mekach taos, and even to Rav Yehuda, a positive mitzvah that was overlooked for a marriage is definitely a marriage, it is only a mekach taos for merchandise because you do not care for that exact object or piece, but you will be satisfied with a different object, and if one has a defect you would prefer a different one without the defect. But for shidduchim, "which are difficult as splitting the Red Sea" when the parties consent to the marriage it is a very happy time, and they would not consent to marry just anyone, only someone who finds favor in their eyes, this would not be considered a blemish to nullify the marriage unless it was a complete defect that, had they known, they never would have agreed to the match.
Regarding the second question, if he is required to disclose his medical situation as to not violate onaah, Shulchan Aruch (C"M 228:6) says it is prohibited to trick people whisle buying and selling, or to mislead them (lignov daatam) for example, if the item has a defect he must disclose ite, even a defect that would not nullify the sale, one still violates "gneivas daas" (see Sma there), if so, in this case he needs to disclose his condition. [This would prevent him from getting married because early in the process every small matter can ruin the match].
However, there seems to be a strong proof he does need to disclose it from Yevamos 45 that we hold a non-Jewish man or slave and a Jewish woman's child is kosher (not a mamzer) but people would not marry such a person. Even Rav Yehuda paskened this person was allowed to marry into the kahal, but he said 'zil itmar' - Rashi explains go to a place where they don't know you and marry a bas yisrael, because if they know you they wouldn't let you marry [a bas yisrael].. Rav said go to a different place or marry someone like yourself (from a union of a non-Jewish man and a Jewish woman) because if not he will not find a wife; even if he was a scholar like Yehoshua bin Nun they would not marry him. Still, in a different place he could marry a bas Yisrael without him telling them (disclosing) his status, for if he would disclose it they would not accept him as a husband, so we see it is permitted to not disclose his defect since he is permitted midioraissa.
And the reason this is not considered misleading (gneivas daas) is like we said above; for merchandise even for an insignificant defect had the merchant refunded his money he would accept it [in exchange for a non-defective item], but for marriage, after the marriage already took place (bediavad) even if both sides would consent to divorce, there is an umdena they do not want that, after they already emotionally bonded, and it is difficult to divorce, and who knows what the future will hold [to find a new spouse] so in any case like this where they will overlook the defect after the fact is not considered gneivas daas even though he did not disclose all defects.
It is also possible that gneivas daas [not an active misleading, but by not disclosing limitations] is only a rabbinic law [not Biblical]. Sefer Chareidim counts it as Rabbinic Mitzvah in number 84 from Negative Rabbinic Mitzvos (midivrei kabbalah). When there is a worry that they will not procreate and other stumbling blocks, the Rabbis did not decree to avoid gneivas daas. Or for some other reason. We see from Yevamos 45 that one does not have to disclose the defect as long as the defect does not cause a violation of Biblical [negative] law.
That which you responded that perhaps only in cases like Yevamos 45 where there was a question of lineage, which was common to onvestigate, if he went to a new place and they did not ask him about his lineage we see they are not so concerned about it and leading themselves into doubtful situations. But in the case of a possible ptzua dakah which is not something people are concerned about (because of its rarity) when they don't ask we cannot assume that they are not concerned about it. This line of reasoning is found in Tosfos (Chullun 94b s.v. inhu), but it is not universally accepted. Maggid Mishna (Mechira ch. 3) brings an opinion if the purchaser would be able to immediately find out [for example, a barrel of wine that he could have tasted] and he did not, and the merchant sold it to him without any warning or disclosure, the sale is final. Mishna Lamelech says Rif and Rambam argue on that opinion. To Rif, in the case of Yevamos where he went to a different place and married there and they did not ask him about his lineage, it does not tell us that those people are not concerned about marrying the child of a Jewish woman and non-Jewish man. [And his actions are not considered misleading since according the the law he is allowed to marry a Jewish woman.] The opinions who hold that when the matter could have been clarified but the purchaser declines to find out it is not a deceptive sale, do not hold that those people were okay with that matter because they didn't ask about it, because really they are concerned, but they did not think they had to worry that this person's father was not Jewish. Because this form of gneivas daas is prohibited only Rabbinically, in an extreme case like this we can rely on Rif and Rambam, as seen inYevamos 45, that he does not have to disclose [the medical issue of the removed testicle]. It also is not evident that not asking a question about lineage shows acceptance of that fact.
That which you mentioned that maybe he is allowed to marry because of the sfeik sfeika, but maybe in reality he is sterile, and sterility is a major defect which would nullify the marriage, since we rely on the Torah principle of Sfeik sfeika that he is not a ptzua dakah, and even Rabbeinu Tam admits we commonly see people like this having children (see Ramban Yevamos 75 and other Rishonim there) and in our times the doctors agree he will be able to father children, if the woman would have known his condition she would have accepted him and believe he would be able to father children, this is not a mekach taus for her.
[This chapter was written as a discussion, without proper investigation (of the Responsa) and I do not say this as a psak (Halachik decision), and those who study this should keep that in mind. Later I saw Shut Chavos Yair 221 about a eunoch marrying, but if you read it, it has no connection to our case.]
