In the Philippines, marriage is protected as an "inviolable social institution." Consequently, the legal system provides stringent mechanisms to address breaches of marital fidelity. Extra-marital affairs are not merely private indiscretions; they carry significant criminal, civil, and administrative consequences under the Revised Penal Code (RPC), the Family Code, and Republic Act No. 9262.

I. Criminal Liability: Adultery vs. Concubinage

The Philippine Revised Penal Code distinguishes between infidelity committed by a wife and that committed by a husband. This distinction has long been a subject of debate regarding gender equality, but it remains the current letter of the law.

1. Adultery (Article 333, RPC)

Adultery is committed by any married woman who has sexual intercourse with a man not her husband, and by the man who has carnal knowledge of her knowing her to be married.

Threshold of Proof: A single act of sexual intercourse is sufficient to constitute adultery.
Penalty: Prision correccional in its medium and maximum periods (2 years, 4 months, and 1 day to 6 years).
Key Feature: Each individual act of sexual intercourse can be treated as a separate crime.

2. Concubinage (Article 334, RPC)

Concubinage is committed by a married man who shall:

Keep a mistress in the conjugal dwelling;
Have sexual intercourse, under scandalous circumstances, with a woman who is not his wife; or
Cohabit with her in any other place.

Threshold of Proof: Unlike adultery, a single act of intercourse is generally insufficient. The prosecution must prove one of the three specific conditions mentioned above.
Penalty: For the husband, prision correccional in its minimum and medium periods (6 months and 1 day to 4 years and 2 months).
The Concubine: The penalty for the mistress is destierro (banishment from a specific radius for a set period), which is a lighter penalty than that imposed on the male adulterer.

II. Infidelity as Psychological Violence (RA 9262)

The Anti-Violence Against Women and Their Children Act of 2004 (VAWC) has revolutionized how marital infidelity is prosecuted. Under this law, "psychological violence" includes acts that cause mental or emotional anguish, specifically mentioning "marital infidelity."

Impact: Many practitioners now prefer filing cases under RA 9262 rather than Concubinage because the penalties are higher and the "scandalous circumstances" or "cohabitation" requirements of the RPC are bypassed by proving "emotional anguish."
Protection Orders: A victim-spouse can seek a Permanent Protection Order (PPO) to remove the offending spouse from the home and ensure the safety of the family.

III. Civil Consequences and the Family Code

While criminal cases seek imprisonment, civil actions address the status of the marriage and the distribution of assets.

1. Legal Separation (Article 55)

Sexual infidelity or perversion is a ground for Legal Separation.

Effect: The spouses are entitled to live separately, but the marriage bond is not severed (they cannot remarry).
Forfeiture: The offending spouse may lose their share of the net profits of the conjugal partnership or absolute community property.

2. Declaration of Nullity (Article 36)

While infidelity itself is not a direct ground for annulment, "habitual infidelity" is often used as a primary symptom to prove Psychological Incapacity. The court may view the inability to remain faithful as a manifestation of a deep-seated psychological condition that prevents the spouse from complying with essential marital obligations.

IV. Property, Inheritance, and Custody

The legal repercussions extend to the financial and parental spheres of life:

Disinheritance: Under the Civil Code, a spouse may be legally disinherited if they have given cause for legal separation (such as adultery or concubinage).
Donations: Any donations made by the offending spouse to the paramour are considered void under Article 739 of the Civil Code, as they are contrary to public policy and morals.
Custody of Children: In cases of legal separation, the "innocent spouse" is generally awarded custody of the minor children. The court applies the "Best Interests of the Child" rule, and while infidelity does not automatically disqualify a parent, it is a significant factor in assessing moral fitness.

V. Administrative Liability for Professionals

For individuals in specific professions, an extra-marital affair can end a career:

Government Employees: Under Civil Service rules, "disgraceful and immoral conduct" is a ground for disciplinary action, which can lead to suspension or dismissal from service.
Lawyers and Judges: The Supreme Court maintains strict ethical standards. Infidelity often leads to disbarment or suspension for violating the Code of Professional Responsibility and Accountability (CPRA), as it reflects poorly on the fitness of the individual to practice law.

Legal Note on Pardon: In criminal cases of Adultery or Concubinage, the offended spouse cannot institute the criminal complaint if they have consented to the offense or pardoned the offenders. A pardon, whether express or implied (such as continued cohabitation after discovery), effectively bars criminal prosecution.