The 53rd prohibition is that we are forbidden from marrying male descendants of Ammon and Moav — even after they have converted to Judaism.

The source of this commandment is G‑d's statement,1 "An Ammonite or Moabite [man] may never enter the congregation of G‑d."

One who transgresses this prohibition receives lashes — i.e. when a male Ammonite or Moabite convert has relations with a Jewish woman within marriage, both receive lashes by Torah law.

The details of this law are explained in the eighth chapter of Yevamos2 and the end of Kiddushin.3