This is the section.
So, if you marry someone in another country, have a child with them, your spouse is denied entry/residency, and the child stays with them, then the child can be denied citizenship.
A. General Requirements: Genetic, Legitimated, or Adopted Child Automatically Acquiring Citizenship after Birth A child born outside of the United States automatically becomes a U.S. citizen when all of the following conditions have been met on or after February 27, 2001:
• The child has at least one parent, including an adoptive parent3 who is a U.S. citizen by birth or through naturalization;
• The child is under 18 years of age;
• The child is a lawful permanent resident (LPR); and
• The child is residing in the United States in the legal and physical custody of the U.S. citizen parent.
So, if you marry someone in another country, have a child with them, your spouse is denied entry/residency, and the child stays with them, then the child can be denied citizenship.