1) Avraham, your question is asked by the Gemara in Yevamos 76b!

The Gemara relates what happened when David went out to fight against Golyas, the giant Pelishti. Shaul asked Avner who David was, and the Gemara recounts the conversation. Then Doeg the Edomite intervened and said, "Instead of asking if David is worthy of being the king, first ask if he is allowed to marry into the Jewish people!" Avner replied that we have learned that he is, since we rule "Mo'avi but not Mo'avis [is disqualified from marrying into the Jewish people]" so David's ancestor Ruth was permitted to the Kahal. To that, Doeg countered, "If so... we should say Mitzri v'Lo Mitzris!"

Avner replied that Amoni and Mo'avi are different, because the verse there (Devarim 23:5) states that the reason they are forbidden is that "they did not greet you with bread and water." It is only the way of men to go out and greet but not the way of women, so this is why Amonis and Mo'avis are permitted.

Since the reason why the first two generations of Mitzrim are forbidden (see Devarim 23:9) is not connected to the fact that they did not greet us, it follows that there is no difference between Mitzri and Mitzris.

2) The Derashah in Yevamos 76b seems to be the opposite of the simple meaning of the verse:

There is something about the above Gemara in Yevamos that has been bothering me a lot and, bs'd, I found that the Sho'el u'Meshiv also had this question, so I will cite what he says.

a) This is in Teshuvos Sho'el u'Meshiv (third edition, part 1, #437). The Sho'el u'Meshiv writes that he is very perplexed at the question of the Gemara (six lines from the bottom of 76b), "Ela me'Atah... Mitzri v'Lo Mitzris." (It is interesting to note that the Sho'el u'Meshiv calls this the question of the Gemara, but in contrast I found, bs'd, that the Or ha'Chayim to Devarim 23:3 calls this the question of Doeg.)

The Sho'el u'Meshiv writes that this sort of Derashah fits well with Amoni and Mo'avi because the verse there (Devarm 23:4) states that they are forbidden and the Derashah of "Amoni and not Amonis" excludes the females from the prohibition. In contrast, about the Mitzri the Torah states, "You shall not despise a Mitzri" (Devarim 23:8). If we make the Derashah "Mitzri v'Lo Mitzris" this would mean that the females are not included in the Heter of the Mitzrim after three generations. The Sho'el u'Meshiv proceeds to prove that it is not possible to say that anyone maintains that Mitzri women are forbidden even after three generations. Without going into all of his proofs for this, we can note (and, in fact, the Sho'el u'Meshiv himself writes this) that Rebbi Shimon in the Mishnah (76b) maintains that the Mitzri women are permitted immediately and Rebbi Yehudah agrees that at least after three generations they are permitted.

b) I also found, bs'd, that the Malbim on Devarim 23:4 -- even though I did not find that he mentions our Gemara -- also writes that one cannot say "Mitzri v'Lo Mitzris." This is because the prohibition of Mitzri for the first three generations is not learned from "Do not despise a Mitzri." On the contrary, from "Do not despise" we learn not to be distant from the Mitzrim. The fact that they are forbidden for the first three generations is learned from Devarim 23:9, "Children who will be born to them for the third generation may enter the Kahal of Hash-m," which implies that the first two generations are forbidden.

c) I have not yet managed to understand the answer of the Sho'el u'Meshiv to this question, so I will leave it as a question for the moment.

3) The Korban ha'Eidah says a Pshat in "Mitzri v'Lo Mitzris" exactly opposite to the Pshat of the Sho'el u'Meshiv:

I found, bs'd, that the Talmud Yerushalmi also asks why do we not say the Derashah of Mitzri v'Lo Mitzris. This is on the Mishnah (cited in Talmud Bavli 76b) that Amoni and Mo'avi are forbidden forever, but the females are permitted immediately. The Talmud Yerushalmi states right away that this is because of the Derashah "Amoni and not Amonis, Mo'avi and not Mo'avis." The Yerushalmi asks on this: Why do we not say a similar Derashah, "Edomi and not Edomis, Mitzri and not Mitzris"? The Yerushalmi answers the same answer to this question that the Bavli gives -- that the Torah states that Amoni and Mo'avi are forbidden because they did not greet us and give us bread and water, while the way of women is not to greet. Since this reason does not apply by Mitzri, the same Derashah could not be made.

The Korban ha'Eidah, on the Yerushalmi, explains that the question of the Yerushalmi that we should say Mitzri and not Mitris means . If we would say Mitzri and not Mitzris, this would mean that there is absolutely no prohibition on the Mitzris after she converted to Judaism; even in the first two generations they should be allowed. This is indeed the opinion of Rebbi Shimon in the Mishnah on 76b.

It seems that according to the Korban ha'Eidah we learn the verse "do not despise a Mitzri" to mean that one may not despise a Mitzri for the first two generations and then from the third generation onwards a Bas Yisrael may marry even a Ger Mitzri. This all applies to males, while the verse does not discuss females at all. There is no requirement to wait at all before marrying the Mitzris convert.

This is the opposite view from the Sho'el u'Meshiv cited above, that if one would say Mitzri and not Mitzris this would mean that the females are always forbidden.

To summarize, the Bavli and the Yerushalmi both ask that we should say Mitzri and not Mitzris and they both answer that the reason we do not say this Derashah is that we say it only for Amoni and Mo'avi because the way of men is to greet, which is not applicable to Mitzri. However, the Sho'el u'Meshiv and Korban ha'Eidah give opposite explanations of what the Derashah would have meant if we would have made it.

4) Here are two more answers to this question from the Rishonim:

a) This question is asked explicitly by Tosfos in Yevamos 77a (DH ka'Tani). The Gemara there (Yevamos 77a) and Rashi (DH ka'Tani) say that there is a dispute among the Tana'im about whether we learn the Din -- that male Amonim are prohibited and females are permitted -- from the Derashah "Amoni but not Amonis" (this is Rebbi Yehudah's opinion) or from the fact that it is only the way of men to greet with bread and water (this is Rebbi Shimon's opinion). Tosfos asks on Rebbi Yehudah that since he learns "Amoni and not Amonis" why does he not also learn Mitzri and not Mitzris?

b) Tosfos gives a lingustic answer in the name of the Ri. The reason why Chazal made a Derashah from the word "Amoni" and "Mo'avi" is because the Torah could have written this in a shorter way. It could have said "Amon" without a Yud at the end, and "Mo'av" without a Yud at the end. These two letters Yud are superfluous, so a Derashah can be made from them. "Amoni" excludes "Amonis" and "Mo'avi" excludes "Mo'avis."

In contrast, the Yud at the end of "Mitzri" is not superfluous. On the contrary, "Mitzri" is the shortest possible way of writing it, because the alternative -- to write "Mitzrayim" (which is the name of the nation, in the same way that Amon and Mo'av are names of nations) -- is longer. Therefore, we cannot make a Derashah "Mitzri and not Mitzris" because there is no extra letter in the word "Mitzri."

c) Tosfos Yeshanim #1 (printed in the margin of the Gemara) gives a different answer (which is also given by the Chidushei ha'Ramban). The answer is based on the Gemara below, three lines from the end of 77b. Rebbi Yehudah learned from Devarim 23:9, "Children who will be born to them in the third generation," that the Torah makes it dependent on birth. Rashi (DH Asher) writes that all children who are born, including females, are permitted only in the third generation. It follows that the verse, "Children who will be born...," reveals that the females are forbidden in the same way as males are, so we cannot say the Derashah "Mitzri and not Mitzris."

Dovid Bloom