In the Philippines, the sanctity of marriage is a constitutionally protected social institution. Consequently, acts of marital infidelity are not merely private wrongs but are categorized as criminal offenses under the Revised Penal Code (RPC). However, the law does not treat infidelity symmetrically; it distinguishes between the acts committed by a wife and those committed by a husband through the crimes of Adultery and Concubinage.

I. Criminal Recourse: Adultery vs. Concubinage

The legal threshold and definitions for infidelity vary significantly depending on the gender of the offending spouse.

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.

Evidence Required: 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).

2. Concubinage (Article 334, RPC)

Concubinage is committed by a married man under specific, more stringent conditions. Unlike adultery, simple sexual intercourse outside of marriage does not constitute concubinage. It is committed only if the husband:

Keeps a mistress in the conjugal dwelling;
Has sexual intercourse under scandalous circumstances; or
Cohabits with her in any other place.

Evidence Required: Proof of "cohabitation" (living together as husband and wife) or "scandalous circumstances" is necessary.
Penalty: Prision correccional in its minimum and medium periods (6 months and 1 day to 4 years and 2 months) for the husband. The concubine is only served a penalty of destierro (banishment from a specific radius).

II. The Rule of Prosecution

Under Article 344 of the RPC, these crimes are considered "private crimes." This imposes specific procedural requirements:

Complainant: Only the offended spouse can file the complaint.
Inclusion of Both Parties: The offended spouse must include both the guilty spouse and the paramour in the complaint, if both are alive.
Pardon/Consent: The case cannot be pursued if the offended spouse has consented to the infidelity or has pardoned the offenders (either expressly or impliedly).

III. Civil and Administrative Remedies

Beyond criminal prosecution, there are several civil avenues to address infidelity.

1. Legal Separation (Article 55, Family Code)

Sexual infidelity or perversion is a ground for legal separation.

Effect: The spouses are allowed to live separately and the absolute community or conjugal partnership is dissolved.
Constraint: The marriage bond is not severed; neither party can remarry.

2. Declaration of Nullity (Article 36, Family Code)

While infidelity itself is not a direct ground for annulment, "Psychological Incapacity" often uses habitual infidelity as a symptom. If it can be proven that the infidelity stems from a grave psychological condition existing at the time of the celebration of the marriage, the marriage may be declared void ab initio.

3. Psychological Violence (R.A. 9262)

Under the Anti-Violence Against Women and Their Children Act (VAWC), marital infidelity can be classified as a form of psychological violence. It causes mental or emotional suffering to the wife, and the husband can be subjected to protection orders and imprisonment.

IV. Legal Consequences for the "Mistress" or "Paramour"

Remedy	Consequence for the Third Party
Criminal (Adultery)	Imprisonment (same as the wife).
Criminal (Concubinage)	Destierro (banishment).
Civil (Article 26, Civil Code)	The offended spouse may file for damages against the third party for meddling with or disturbing the family relations of another.
Article 201, RPC	If the relationship is "scandalous," they may be liable for crimes against public morals.

V. Disqualification and Forfeiture

Proven infidelity carries heavy financial and professional consequences:

Forfeiture of Benefits: The guilty spouse loses their share of the net profits of the conjugal properties upon legal separation.
Succession: The guilty spouse may be disqualified from inheriting from the innocent spouse by way of intestate succession.
Custody: While the "tender age" rule generally favors the mother for children under seven, "moral depravity" stemming from open infidelity can be used to challenge custody.
Administrative Liability: For professionals (lawyers, doctors, government employees), "immorality" or "disgraceful/immoral conduct" is a ground for disbarment or dismissal from service.

VI. Summary Table of Key Differences

Feature	Adultery (Wife)	Concubinage (Husband)
Number of Acts	One act is enough.	Must be continuous/scandalous.
Location	Anywhere.	Specific (Conjugal home/cohabitation).
Penalty for Spouse	2yr 4mo to 6yr.	6mo 1day to 4yr 2mo.
Penalty for Partner	Same as wife.	Banishment (Destierro) only.