In the Philippines, Adultery is classified as a Crime Against Chastity under the Revised Penal Code (RPC). Unlike many jurisdictions where infidelity is strictly a civil matter for legal separation or divorce, the Philippine legal system maintains criminal penalties for marital infidelity.

1. Legal Definition and Elements

Under Article 333 of the Revised Penal Code, Adultery is committed by any married woman who shall have sexual intercourse with a man not her husband, and by the man who has carnal knowledge of her knowing her to be married, even if the marriage be subsequently declared void.

To secure a conviction for Adultery, the following elements must be proven beyond reasonable doubt:

The woman is legally married to the complainant.
The woman has sexual intercourse with a man other than her husband.
The paramour (lover) has sexual intercourse with her while knowing she is married.

2. Adultery vs. Concubinage

It is critical to distinguish Adultery from Concubinage (Article 334), which is the crime applicable to a married husband. The legal standards differ significantly:

For Adultery: A single act of sexual intercourse is sufficient to file a case.
For Concubinage: The husband must have kept a mistress in the conjugal dwelling, had sexual intercourse under scandalous circumstances, or cohabited with her in any other place.

3. Who Can File the Case?

Under the Rules of Court, the crime of Adultery is a private crime. This means it can only be prosecuted upon a complaint filed by the offended spouse (the husband).

The husband cannot file the case against only one party; he must include both the wife and the lover in the complaint, provided both are alive.
If the husband has consented to the adultery or pardoned the offenders (either expressly or impliedly), the criminal action cannot prosper.

4. Required Evidence

Since sexual acts are rarely performed in public, "smoking gun" evidence is not always required. The court often relies on circumstantial evidence, provided the circumstances lead to a fair and reasonable conclusion of illicit activity. Common forms of evidence include:

Testimonies of witnesses (e.g., neighbors, private investigators).
Photographs, videos, or CCTV footage showing intimacy or shared lodgings.
Love letters, SMS, or social media chats (subject to the Rules on Electronic Evidence).
Birth certificates of children born during the period of the alleged affair, where the husband is not the father.

5. Penalties and Consequences

Adultery carries the penalty of prision correccional in its medium and maximum periods.

Imprisonment: Generally ranges from 2 years, 4 months, and 1 day to 6 years.
The Lover’s Penalty: If the lover knew the woman was married, he suffers the same prison sentence. If he did not know she was married, he cannot be held criminally liable for adultery.
Civil Effects: A conviction for adultery is a legal ground for Legal Separation (Article 55 of the Family Code). It may also result in the guilty spouse’s disqualification from inheriting from the innocent spouse and the loss of her share in the profits of the conjugal partnership.

6. Defenses and Bars to Prosecution

Accused parties typically employ the following defenses:

Pardon or Consent: If the husband knew of the affair and continued to live with his wife or explicitly forgave her, the case can be dismissed.
Prescription: The crime of adultery prescribes in 10 years from the time the offended party learned of the crime.
Death: The death of either the wife or the lover before final judgment extinguishes their respective criminal liability.

7. Procedural Step-by-Step

Affidavit-Complaint: The husband prepares a sworn statement detailing the acts and attaches supporting evidence.
Preliminary Investigation: Filed at the Prosecutor’s Office in the city or province where the sexual act occurred.
Filing of Information: If the Prosecutor finds "probable cause," a formal "Information" is filed in the Regional Trial Court (RTC).
Warrant of Arrest: The court issues a warrant; however, Adultery is a bailable offense.
Trial: The prosecution must prove the illicit intercourse occurred through direct or circumstantial proof.