The heathens esteemed fornication either no sin, or a very little one; but adultery was heinous and formidable even among the heathens, and especially here, because it was fresh in memory how sorely God had punished Abimelech, and all his family, only for an intention of adultery, Genesis 20:1-18. Note here, they take it for granted that their ignorance had not been a sufficient excuse for their sin.
Related Commentaries of Genesis 26:10
Genesis 26:10
10 And Abimelech said, What is this thou hast done unto us? one of the people might lightly have lien with thy wife, and thou shouldest have brought guiltiness upon us.