In the Philippines, marital infidelity is not merely a moral failing or a ground for legal separation; it is a criminal offense classified as a Crime Against Chastity under the Revised Penal Code (RPC). However, the law treats infidelity differently based on the gender of the offending spouse, distinguishing between Adultery and Concubinage.

1. Adultery (Article 333, RPC)

Adultery is committed by a married woman and the man who has carnal knowledge of her, knowing her to be married.

Key Elements:

The Offender: A married woman.
The Act: A single act of sexual intercourse is sufficient to constitute the crime.
The Paramour: The man involved is also liable if he knew the woman was married at the time of the act.

Penalty:

The penalty for the guilty parties is prision correccional in its medium and maximum periods (typically 2 years, 4 months, and 1 day to 6 years).

2. Concubinage (Article 334, RPC)

Concubinage is committed by a husband who keeps a mistress. Unlike adultery, the mere act of sexual intercourse by a married man is not enough to sustain a criminal conviction.

Grounds for Concubinage:

To convict a husband, the prosecution must prove one of the following specific circumstances:

Keeping a mistress in the conjugal dwelling: The husband brings the woman into the home he shares with his wife.
Sexual intercourse under scandalous circumstances: The relationship is public, notorious, or flaunted in a way that causes public offense.
Cohabitation: The husband lives with his mistress in any other place (e.g., a separate apartment or "second home").

Penalty:

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 is destierro (banishment), meaning she is prohibited from entering a designated radius (25km to 250km) from the victim's residence for a specific period.

3. Procedural Requirements and Limitations

The Rule of Double Prosecution

A complaint for Adultery or Concubinage cannot be filed against one party only. The offended spouse must include both the offending spouse and the paramour/concubine in the same complaint, provided both are alive.

Who Can File?

Only the offended spouse has the legal standing to file the criminal complaint. It is considered a private crime, meaning the State will not prosecute it without the initiative of the victimized spouse.

Bars to Prosecution:

Consent: If the offended spouse agreed to the infidelity or the "sharing" of the spouse.
Pardon: If the offended spouse has expressly or impliedly (by continuing to live/sleep with the offender) pardoned the infidelity. Once a pardon is given, the right to file a criminal case is extinguished.

4. Evidentiary Standards

Because these are criminal cases, the standard of proof is Proof Beyond Reasonable Doubt.

Direct Evidence: Rare, as parties seldom engage in these acts publicly.
Circumstantial Evidence: Courts often rely on a "combination of circumstances" that, when taken together, leave no room for doubt that the acts occurred. This includes love letters, photographs, travel records, birth certificates naming the husband as the father of a child with another woman, and testimonies of witnesses.

5. Infidelity as Ground for Legal Separation (Civil Aspect)

While Adultery and Concubinage are criminal, "sexual infidelity" is a ground for Legal Separation under the Family Code of the Philippines.

Effect: A decree of legal separation allows the spouses to live apart and dissolves the absolute community of property or the conjugal partnership.
Limitation: It does not dissolve the marriage bond. The parties are still legally married and cannot remarry.

6. Psychological Incapacity (Article 36, Family Code)

Frequent or habitual infidelity is often used as a "manifestation" of Psychological Incapacity. While infidelity itself is not the incapacity, it can be evidence of a spouse’s inability to comply with the essential marital obligation of fidelity, which may lead to a declaration of Nullity of Marriage (voiding the marriage from the beginning).

Comparison Summary Table

Feature	Adultery (Wife)	Concubinage (Husband)
Required Act	Single act of intercourse.	Cohabitation, keeping in the home, or scandalous intercourse.
Proof Difficulty	Relatively lower (intercourse focus).	Higher (must prove living arrangements or scandal).
Primary Penalty	Imprisonment.	Imprisonment.
Paramour Penalty	Imprisonment.	Banishment (Destierro).
Pardon	Bars prosecution.	Bars prosecution.