Deuteronomy 23:4 "An Ammonite or a Moabite shall not enter the congregation of G‑d"

We are not allowed to marry a man from the nations of Ammon or Moab even if he has converted to Judaism.

However, it is permissible to marry a woman from Ammon or Moab after she has converted to Judaism.

These nations refused to offer bread and water to the Jewish people when they passed by their borders and because of this act of cruelty, they were excluded from ever being able to join the Jewish people in marriage.