Yevamot 76 - The congregation of Israel
We mentioned earlier that one who has his testicles wounded or crushed, and also one whose member is severed cannot "enter the congregation of Israel," that is, he cannot marry a Jewish woman who is born as a Jew. However, he can marry a convert. Although converts are Jewish in every respect, they are not encompassed by the term "congregation." Their children, however, will be called "congregation."
There is also another view, that of Rabbi Yehudah, according to which converts are also included I the "congregation," and accordingly our man discussed above cannot marry anyone. The source for this law is found in the Torah here .
Similarly, a convert from the people of Amon or Moav is prohibited to marry into the congregation. This was only true when these peoples lived as separate nations. After king Sennacherib expanded his empire and resettled entire peoples, they all got mixed, and the law stopped being applicable. Even when it was applicable, only male converts had the prohibition, but not female, since the Torah formulated the prohibition in terms of "Moavi" - which implies the male, but not "Moaviyah" which would imply a woman.
Art: Dead Sea with the mountains of Moab by Gustave Bauernfeind