Recently I was asked about the issue of Biblical characters having multiple wives in the Old Testament and the inquirer wondered if these were acts of adultery. This is a reasonable question and is worthy of a post.
To the people at the time they probably weren’t considered acts of adultery but objectively speaking (in God’s eyes), these were acts of adultery. The Catechism, based on the words of Jesus, notes that “polygamy is not in accord with the moral law” (Paragraph 2387). Since polygamy is a violation of the moral law, a law that does not change, it was always objectively adultery to have multiple wives. We should keep in mind that in the days of the Patriarchs (the people of God in the days of Genesis) polygamy was very common and was not explicitly rejected because God’s people were not ready for such a radical departure of the common customs of the time, just as Moses permitted divorce due to the hardness of the people’s heart yet it was never God’s original plan. The Catechism paragraph 1610 touches on this matter briefly, “In the Old Testament the polygamy of patriarchs and kings is not yet explicitly rejected. Nevertheless, the law given to Moses aims at protecting the wife from arbitrary domination by the husband, even though according to the Lord’s words it still carries traces of man’s “hardness of heart” which was the reason Moses permitted men to divorce their wives.”
So, objectively they were in sin, but their moral culpability was probably significantly diminished due to the fact that there wasn’t an explicit commandment at the time prohibiting it and thus they did not have full knowledge and consent of the will in committing the sin.