In the Philippines, marital infidelity is not merely a moral transgression but a criminal offense. However, the Revised Penal Code (RPC) treats infidelity differently depending on whether the offending spouse is the husband or the wife. These crimes are classified as Crimes Against Chastity and require a private party (the offended spouse) to initiate the prosecution.

1. Adultery (Article 333, Revised Penal Code)

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.

The Act: A single act of sexual intercourse is sufficient to constitute adultery.
The Parties Liable: Both the wife and her lover are criminally liable.
Evidence: Since sexual acts are usually done in private, direct evidence is not always required. Circumstantial evidence—such as staying in a hotel room together or intimate letters—can be sufficient if it leads to an unavoidable conclusion of carnal relations.
Penalty: Prision correccional in its medium and maximum periods.

2. Concubinage (Article 334, Revised Penal Code)

Concubinage is committed by a married man who keeps a mistress in the conjugal dwelling, or shall have sexual intercourse under scandalous circumstances with a woman who is not his wife, or shall cohabit with her in any other place.

Unlike adultery, a single act of intercourse is not enough to convict a husband of concubinage. To file a case, the wife must prove one of the following three specific legal grounds:

Keeping a mistress in the conjugal dwelling: The husband brings the other woman into the family home.
Sexual intercourse under scandalous circumstances: The relationship is so public and flaunted that it shocks the common sense of decency in the community.
Cohabitation: The husband and the mistress live together as husband and wife in a different location (e.g., a "second home").

The Parties Liable: The husband and the concubine.
The Penalty: The husband faces prision correccional in its minimum and medium periods. The concubine, however, is only given the penalty of destierro (banishment from a specific radius of the complainant's residence).

Key Differences at a Glance

Feature	Adultery (Wife)	Concubinage (Husband)
Who commits it?	Married woman	Married man
Standard of Proof	Single act of intercourse	Strict grounds (Cohabitation, scandal, or home-sharing)
Penalty for Lover	Imprisonment	Banishment (Destierro)
Difficulty to Prove	Relatively easier	High burden of proof

3. Common Legal Requirements and Defenses

Regardless of whether the charge is adultery or concubinage, certain procedural and substantive rules apply:

Who can file? Only the offended spouse can file the complaint. Both the guilty spouse and the paramour must be included in the same complaint (they cannot be sued separately).
Pardon and Consent: The case cannot be prosecuted if the offended spouse has consented to the infidelity or has pardoned the offenders (either expressly or impliedly). "Implied pardon" often occurs if the couple continues to live together and have sexual relations after the discovery of the affair.
Death of a Party: If the offending spouse or the paramour dies before a final judgment, the criminal liability is extinguished.

4. The Impact of the "Safe Spaces Act" and Gender Equality

There is ongoing legal debate regarding the disparity between these two laws. Critics argue that the higher burden of proof for concubinage is discriminatory against women. While the RPC remains the law of the land, victims of infidelity often utilize Republic Act 9262 (Anti-Violence Against Women and Their Children Act) as an alternative. Under RA 9262, marital infidelity can be categorized as Psychological Violence, which is often easier to prove and carries significant penalties.

5. Jurisdictional Requirement

These cases must be filed in the Regional Trial Court or Municipal Trial Court of the city or municipality where any of the essential elements of the crime took place (where the intercourse or cohabitation occurred).