Oh, adultery is a crime in some states. I'm not denying that. I'm saying that if it were a breach of contract, it wouldn't be a crime.
Now you're not even making sense.
From a Libertarian Perspective, the only Actions which should be Legally Punished as "Crimes" are precisely those Actions which Harm or Defraud another Human Being (you know... Aggression, Perfidy, Theft, Fraud, Despoilment; in other words, exactly the Second Table of the Ten Commandments) -- Theonomic Libertarianism simply represents an virtuous improvement over Secular Libertarianism in recognizing the Contract of Marriage as the most Socially-Fundamental and Justiciable of All Private Contracts.
But surely you are not saying that "breach of contract wouldn't be a crime"!!
Without the freely-willed Organization of Accountability known as the "Private Contract", Man has no modus of social interaction and no recourse for justice -- except the Totalitarianism of the State!
Surely you must agree that -- in order for a Voluntary, Non-Totalitarian Civil Society to exist -- Private Contracts MUST be Sacrosanct, and Breach-of-Contract MUST be Against the Law.
Would you not agree?