The Catechism of Trent, issued by Pope St. Pius V, says that the patriarchs of the Old Testament are not to be blamed for their polygamy. Therefore sex other than monogamous baby making is not directly contrary to natural law. If a person is not feeling the grace of the New Testament era, can they be blamed for not keeping their sexual activities for one spouse?
Different people feel different grace in different aspects of life. Right?
What a person “feels” is irrelevant.
Polygamy is contrary to divine positive law governing the marriage union (Genesis 2:24; Ephesians 5:31). According to the natural law, even successive polygamy (as in societies that legalize marriage after divorce) hinders the proper care and education of children. And it places an intolerable burden on practicing mutual love between the spouses.
In the Old Testament, God tolerated polygamy for a certain time, as it appears from the examples of men such as Abraham, Jacob, and David. But with the proclamation of the New Law, this concession, almost wrested from by God by reason of the moral obtuseness of man, was revoked. Marriage was restored to its original unity. The language of Christ is very explicit (Matthew 19:3-9; Mark 10:1-12; Luke 16:18). Catholic tradition has consistently interpreted Christ’s teaching as absolutely forbidding polygamy, and the prohibition was, in fact actually defined by the Council of Trent, pronouncing anathema against anyone who says that “it is lawful for Christians to have several wives at the same time, and that it is not forbidden by any divine law” (Denzinger 1802).
Your assertion doesn’t follow from your statement. If polygamy weren’t contrary to natural law, then there wouldn’t even be a question of ‘blame’ – there’d be nothing to blame! Therefore, the assertion demonstrates that there is potential blame; and consequently, there is sinfulness here.