Simple version:
Liability for interference in a marital contract requires restitution and punishment. (Imagine if the cost of a divorce was paid for by the interfering parties.)

Long Version:

Adultery is a demonstrably irreciprocal act: it imposes costs on others—especially one’s mate and offspring—without their consent or restitution. In terms of Natural Law, that makes it criminal if unaccounted.

1. From First Principles
Law institutionalizes reciprocity. All crimes are acts of uninsurable imposition of cost on others—whether physical, financial, reputational, or existential.

Marriage is a contract of intergenerational cooperation, primarily to insure against risk (especially for women during childbearing and men against paternity uncertainty).

Adultery violates that contract by introducing external risk (genetic, emotional, economic) without prior disclosure or agreement.

2. Causal Chain of Harm
To the spouse: breach of trust, reputational harm, risk of disease, diversion of resources, emotional destabilization.

To offspring: genetic ambiguity (for males), increased chance of family dissolution, long-term loss of capital (attention, resources, education).

To community: erosion of trust in institutional marriage, weakening of incentives for paternal investment, increase in underclass formation.

Thus, adultery is not merely a private moral failing but a publicly consequential act when viewed as an externalization of costs.

3. Crime or Torts?
If marriage is formalized as a contract with legal obligations (as it should be), adultery constitutes a breach of contract with measurable externalities.

Whether treated as a criminal act (punishable by the state) or a civil tort (subject to restitution) depends on:
Whether the act violates formal institutional commitments (legal marriage).
Whether it causes irreversible harm (e.g. cuckoldry, abandonment).
Whether restitution is possible or sufficient.

In a reciprocal legal order, adultery would:

Be a civil tort if reparable.

Be a crime if the act is concealed, irreparable, or results in parasitic externality (e.g., paternal fraud).

4. Historical Justification

Traditional legal codes treated adultery as criminal precisely because paternity, lineage, and property transmission are foundational to civilization.

Modern liberal regimes, by separating morality from law, tolerate parasitism under the guise of freedom—at the cost of civilizational stability.

5. Position
Adultery is a criminal act under Natural Law if it imposes irreparable, unreciprocated, and concealed costs on others—especially within formal marriage contracts.

It should be:

Prosecutable when the harm is beyond restitution.
Restitutable when damage can be quantified.

Prevented by the clear institutionalization of reciprocal obligations in marriage law.