(a) Your question is asked by the Kehilas Yaakov (Yevamos #44, by the Steipler ZT"L). He answers that there is a crucial difference between getting married and between buying a mundane article. Concerning business matters the Shulchan Aruch CM 228:6 states that if there is a blemish in the merchandise one is obligated to inform the buyer about this. SM"A #7 writes that even if the blemish is not severe enough to enable the purchaser to demand his money back if he was not told about it in advance, nevertheless the seller is obligated in the first place to tell him about it, and if he does not this is considered "Genevas Da'as".

The Kehilas Yaakov argues that the rules concerning getting married are different. This is because in business, even though the customer does not want to be such a particular person to demand his money back for a relatively small defect that he was not told about, nevertheless if the seller would volunteer to cancel the sale, the buyer might be happy to do so. In contrast once a couple are married they do not want to get divorced merely because of some minor problem which they found out about only after the marriage (even if this was a problem which may have caused them to call off the Shiduch if they would have known about it in advance). This is because divorce is so difficult (see the Gemara in Sanhedrin 22a: "How hard divorce is!") and who knows whether he will find such a good wife again?

For instance if one buys a table one does not really care if one recieves this particular table or a similar item. For the same money one is happier to switch it for a different specimen without the drawback of the first. In contrast making a Shiduch is just as difficult as crossing the Dead Sea (see Sotah 2a) and when the two sides manage to agree to get married this is a source of great joy for them. One is not prepared to marry anyone, but only someone one has chosen and who finds favor in one's eyes. Therefore a minor drawback is not sufficient to cancel the marriage, and only a major fault - about which it is known that if the other side would have been aware of it he would not have been willing to get married under any circumstances - is considered a blemish to call off the marriage even after the wedding.

The Steipler gives a golden rule in Shiduchim (incidentally the title of his chapter is "Concerns about cheating in Shiduchim"): If the other side would not want a divorce afterwards because of this particular drawback, then one is not obligated to tell about it in the first place. If one would have to disclose every small drawback, no Shiduch would ever go through.

Therefore in our Gemara, even though people do not want their daughter to marry someone whose father is a Nochri, nevertheless they would not demand a divorce if they only found out about this afterwards. Consequently this is not Genevas Da'as if one did not mention it, and is not a "Mekach Ta'us" if it happened.

(b) Rav Yosef Shalom Elyashiv Shlita (cited in Nishmas Avraham by Prof. A. S. Avraham vol. 5 p.118 DH ACH, and see also notes of Rav Elyashiv Kidushin 71b p. 405) answers that Rav Yehudah ruled that even though the father is a Nochri, nevertheless the child is entirely Kosher and there is no Halachic problem whatsoever in him marrying a Jewess. However the unlearned people in Rav Yehudah's locality did not accept this ruling, so he advised his questioner to go to a different area. Since there is no genuine problem about him getting married there, this is neither Genevas Da'as nor Mekach Ta'us.

KOL TUV

D. Bloom