yitro- Rav Saadiah Gaon: Mitzvah 35 – Concept 160
“You shall not commit adultery.” (Exodus 20:13) We are forbidden to have relations with a married woman. (Rambam, Hilchot Issurei Bi’ah – The Laws of Forbidden Relations) The Rambam derives this law from a different verse: Leviticus 18:20.
Divine Wisdom forbade fornication in order that men might not become like the beasts with the result that no one would know his father so as to show him reverence in return for having raised him.
Another reason for this prohibition was that the father might bequeath unto his son his possessions just as the son had received from his father the gift of existence.
A further reason was that a human being might know the rest of his relatives, such as his paternal and maternal uncles, and show them whatever tenderness he was capable of. (Rabbi Saadiah Gaon, The Book of Beliefs and Opinions 3:2: The Reasons for the Rational Precepts of the Torah and Some of the Motives Behind the Revealed Laws.)