Commentaire sur Le Lévitique 20:10

ואיש AND THE MAN [THAT COMMITTETH ADULERY SHALL SURELY BE PUT TO DEATH] — a man, thus excluding a minor (Sifra, Kedoshim, Chapter 10 8).

את אשת רעהו, excluding if he slept with a “wife” who was a gentile. The adulterer would then not face the death penalty.

ואיש אשר ינאף את אשת רעהו, “and the man who engages in an adulterous relationship with the wife of his fellow, etc.” our sages see in the word איש an exclusion of the death penalty for a minor between the ages of nine and thirteen who sleeps with the wife of a fellow Jew. In the words אשת רעהו they see an exclusion of the death penalty for someone who sleeps with the wife of a Gentile (compare Sanhedrin 52). The words אשת איש indicate that sleeping with the wife of a minor is also not subject to the death penalty. Our verse proves that a marriage ceremony entered into with a Gentile is not considered a legal marriage.